noise problem - when LED-meter inserted. What's going on?

[lion]

I've run into a challenge with a simple AN6884 based LED-meter circuit. I did one similar a few month ago (althought applied differently) and had no problems. Unfortunately the first one isn't in my possion anymore so I can't compare details.

The metering/LEDs works fine – the problems is that the circuit is somehow inducing noise "back into" the signal path/power amp.

EDIT: Drawing error in schem corrected!



The noise is a low freq "ooommmph" sound, which clearly follows the fade out of the LED's.
Nothing when LEDs turns on, or are fully litt – just a "ooomph" of 0.5 -1 sec as each LED turns OFF. The noise is loud enough to be noticeable/annoying even at bedroom levels

The 2N5458 buffer ahead of the AN6884 circuit is a later addition – in the hope that it would "isolate" the LED circuit from the signal path. No change with or without the buffer. Replaced the AN6884 – no change.

2 observations
- The level of the noise is related to the LED brightness adjustment. Low(er) as the brightness of the LEDs are turned down – louder when the LED intensity is high
.
- The noise disappears all together when the diodes/limiter part of the circuit is disconneted (LIM switch OFF).

I could skip and live without the meter – but I'm intrigued, and I won't learn anything by just giving up.

Can anyone enlighten me what's going – and a possible way around it. Thanks.

Erik

[GibsonGM]

What if you inserted a buffer between this circuit and the preamp? Just a thought, and it may be redundant as the 2nd BJT is doing same thing. If it didn't do it with another that you built, I'm kind of at a loss, too...

The 5458 looks like a gain stage to me, rather than a buffer...wonder if it would help if it were re-wired as such (?)

Just thinking out loud.

[karbomusic]

Current draw recovering? It may not be directly getting into the signal but a side-effect from the power rail when all those LEDs light up then release? All a guess but can you test by powering it separately just to see?

[PRR]

It can't work as drawn. The 220K pot really gets signal from the 100uFd main filter cap.

[lion]

Schematic corrected. Sloopy cut & past work - sorry!

The first meter circuit I built was the same (except the buffer stage), but the signal to the LED driver was tapped off at the input of the Thomas Vox limiter circuit - opposed to from the output in this version. I can't see how that would make a difference.

Erik

[R.G.]

Let's think for a minute.
Hmmm. The LED/meter circuit is causing noise back in the main circuit. What circuit elements do these share so that noise could possibly get back there?

Hint: wires are really low value resistors, and capacitors are not really "short circuits".

[lion]

Buffer stage omitted for simplicity



OK, shared circuit elements?
The DC supply (and ground) and ....?

The fact that the problem disappears with the diode/res combo network disconnected (LIM switch OFF) would suggest the noise gets back through this section, but I'm afraid it's over my level what is the source of the noise in the first place.

More hints please (rookie level).

Erik 

[R.G.]

Doh!! For a minute I didn't recognize my old friend, the Thomas Organ variable clipper/limiter. Good on you for using that. It's wildly underappreciated.

But rookie or not, you hit on the problem areas I was intending - power and ground. And you went me one better by figuring out what piece deleted the noise when disconnected. So you're maybe not so rookie as you think. Again, good.

In this circuit, you have to watch where the currents are flowing. The signal currents come through the V+ and ground leads, and so do the switching currents. Since the ground and power leads are actually low-value resistors, changes in ground current look to the circuit like changes in power supply voltage or ground voltage.

I did a similar thing with an LM391? meter circuit, and got a tick/buzz when the LEDs changed. The solution was to separately decouple the metering section and give it its own ground wire so it could not affect power and ground to the signal circuit.

If I were messing with this circuit, I would take the limiter level pot (22K) to the +15V before the 10R and change the 10R to 100R, as well as trying returning the ground from the limiter to power supply ground on a separate wire. I'd do the same for the AN6884. 

However, there are a couple of other places to worry about. The 10uF cap at the limiter switch is backwards for some settings of the 22K. It really should be an NP type. This might be an issue with the circuit. I would also not let the switch open the path to the 10uF; I would put a 1M to the 10uF from the base of the PNP and have the limiter switch shunt it to turn the limiter on. Also, the 3.3K you're using ( to lower gain when the limiter is off?) ought to go on the other side of that 2.2uF output cap, across the "unity" adjuster. Otherwise it causes big changes in the output DC voltage level at the collector of the PNP. That may in fact be the source of your actual problem too.

Try the additional 1M and moving the switch connection for the 3.3K first, because they're easy and don't involve rewiring so much. If you're lucky, that will fix it. Otherwise, dive into rewiring.

[PRR]

> The 10uF cap at the limiter switch is backwards for some settings of the 22K.

...ALL settings of the 22K, as I make it out.

The node at the switch sits at +10V. The threshold level runs to 1.36V max.

The cap "+" mark is just on the wrong side.

The PSRR of this circuit is odd, and maybe not-large. That is of course why (one reason) they have the 10r+100uFd- to filter garbage from the supply.

Erik- when the LEDs come on, they pull big current from the 10r, which causes increased voltage drop. This drop upsets the transistor stages. This drop is buffered by the 100uFd, but that's still a "ooommmph".

I have a big fluffy dog and I need a BIG dog-wash tub. 1-inch hose to get the beast wet. I find a 1/4" pipe under the drinking fountain and tap it. When I open the 1" hose, the drinking fountain hardly dribbles; when I shut the fat hose the fountain squirts my face.

Same if I run the 2-inch dog-tub drain into the 1/2" drain of the drinking fountain. When I empty the tub it backs-up in the drinking fountain drain.

See Image.


The LED chip does not need clean power and its supply should probably come from the far end of the 10r+100uFd.

Likewise for "ground", because those big thumps of LED current are quivering the "ground" that the transistor stages stand on. However you have not "shown the whole circuit" and a lot of little triangles don't make ground path explicit.

Finally, retro-engineer: I bet the 10r (if ONLY supplying the transistors and clipper) could be 100r. Makes little difference to the transistor stages. Makes a big difference to how well the 100uFd can stabilize supply garbage.

_I_ think the 22K trim to the threshold network should come from the 100uFd. Or possibly hang 100uFd from the junction of the 22K and the 1K to stabilize the threshold voltage without upsetting the impedances here.

[lion]

Thanks R.G. and Paul. Almost midnight and I'm too tired to think - tomorrow, hopefully.

Erik

[karbomusic]

when the LEDs come on, they pull big current from the 10r, which causes increased voltage drop. This drop upsets the transistor stages. This drop is buffered by the 100uFd, but that's still a "ooommmph".

OK so I was at least in the right neighborhood. Whew. I remember dealing with something similar in my Vue Meter design. I found it while breadboarding and corrected so I never had to deal with it in the final product.

[lion]

Noise issues solved!

Reworking report.
Upped the 10R to 100R (the filter cap turned out to be 220uF, not 100uF as shown). Repositioned the 3.3k to across the unity trimmer, and added a 1M going to the 10uF on a shunt switch. Paul is right about the orientation of the 10uF – I checked the orig. Thomas Vox schems and the + goes to the top, which is actually also how I had it wired – only for some reason it seems the one schem version I copy/pasted from for illustration simply has I wrong. To follow the recommendation it really should be a NP cap, I didn't have any NP's - instead I put two 22uF's in series + - - +.

I've build the limiter circuit, LED driver and the regulated supply on individual vero's, so while I was at it it wasn't much more work to do the total suggested reworking. So, moved the power feed to both the LED circuit and the treshold pot ahead of the 100R. Grounds from limiter and the AN6884 on separate wires to power supply ground. Also added the suggested extra 100uF from the junction of the 22k and the 1K to stabilize the treshold voltage. After testing, it turned out I had missed soldering the cap ground end on the vero - although it seemed to work without the added stabilization I've soldered and kept it. The transistor voltages measures spot on with Paul's calculations.

I'm glad the noise problems are solved, and also the limiter in/out switching is dead silent now.
In case I've missed or got something wrong, please let me know.

Thanks gent for the help and the very nice educating explanations – much appreciated.

Erik

NB, and a bit OT!
R.G. – re the Thomas Organ limiter, actually this project might be a possible first step towards the Thomas Vox SS amp clone - or sort of - I've been planning to build for years. I've never played any of the TV amps, but the fact that you have kept no secret of how much you like them (despite their seemingly bad rep in other circles) got me curious and interested a long time ago.

Anyhow, at this point the limiter circuit just sits between a Joyo AC Tone amp simulation pedal as prelim preamp, and an old british HH S130 power stage. Although playing guitar since the early 60's this is my first real experience with a solid state amp for guitar. With this simple (and cheap) setup running along side my AC30, the pure valve sound isn't that much better/different (mostly thanks to the Joyo I guess, but anyway). So far I'm mildly surprised - and the point I have proven to myself is that a solely solid state rig can be made to sound quite close to a pure valve amp.

A couple of thoughts, though.
I've often read the comment that "valve watts are louder than SS watts".
From what I've tried so far I have to agree – and it seems not just a subtle difference. The HH S130 I use is suppossedly 130w @ 4 ohms (A quick measurement showed 25 volts output at the point of clipping across a 8 ohm dummy load  – that's > 80w, right?) Even so the SS rig running through a cab of Celestion Greenbacks (not that unsimilar to the AC30's Blue's - is fighting to keep up with my AC30. Interesting.

In that relation - not being used to the "feel" of  a ss guitar amp (loudness vs watts - and how hard the amp is working) I've found the Thomas Vox Limiter to be a nice addition to keep the power stage in the safe area. However it's obviously not a brick wall limiter, and from where the soft limiting start to take place there seems to be region where a sine wave gets increasingly squashed but still with increasing overall level. I take it it's the nature of the circuit – but it has to be taken into account when setting the treshold and the input level to the lim circuit. Yet it doesn't mention in the orig setup procedure (and your recent re-write in the replacement board doc). I'm guessing it has to do with the fact that in the TV amps the preamp levels are a known/incalculated factor - whereas I'm working outside of this and with maybe more drive from the "preamp", if that makes any sense at all?

For the same reason I've found adding the LED meter, with the upper LED adjusted to indicate the the power amp clipping point, a usefull addition – to show where in the safe region the amp is driven (when I haven't got the same "feel" for it as with valve amps), but also showing when peaks starts clipping. Driving the output stage into severa clipping is obviously quite noticeable.

End of long rant.

[R.G.]

Noise issues solved!
[...]

Good work!

re the Thomas Organ limiter, actually this project might be a possible first step towards the Thomas Vox SS amp clone - or sort of - I've been planning to build for years. I've never played any of the TV amps, but the fact that you have kept no secret of how much you like them (despite their seemingly bad rep in other circles) got me curious and interested a long time ago.

Yeah, I have heard lots of people bad-mouth them, but I am, for better or worse, of the generation that heard these things live. New and in good condition, they do sound good. They don't age as well as tube amps, perhaps because they are so very difficult to work on and update to equivalent-to-new, much more so than tube amps.

Anyhow, at this point the limiter circuit just sits between a Joyo AC Tone amp simulation pedal as prelim preamp, and an old british HH S130 power stage. [...] So far I'm mildly surprised - and the point I have proven to myself is that a solely solid state rig can be made to sound quite close to a pure valve amp.

That replicates my experience, as well.

I've often read the comment that "valve watts are louder than SS watts".
From what I've tried so far I have to agree – and it seems not just a subtle difference.

There is some complex psychoacoustics going on. An AES paper by Russell O. Hamm contended that the first 12db or so of clipping in a triode stage produced distortion products that fooled the ear into thinking that the output was louder, and not distorted. Solid state stages tend to suddenly break into hard-edged clipping, so the threshold is not so pleasant a place.

There is no real argument that in terms of actual power, watts is watts - the heat generated in a load resistor is the same for the same electrical input. So any differences have to be in the spectrum of the signals generated. That is, valve watts and SS watts are actually the same, but our ears interpret them differently.

The HH S130 I use is suppossedly 130w @ 4 ohms (A quick measurement showed 25 volts output at the point of clipping across a 8 ohm dummy load  – that's > 80w, right?) Even so the SS rig running through a cab of Celestion Greenbacks (not that unsimilar to the AC30's Blue's - is fighting to keep up with my AC30. Interesting.

25.3Vrms into 8 ohms (resistive) is 80W. However, it is really impossible to compare amplifier loudness from two different speakers. Guitar speakers may vary as much as 20db in the sound output for the same electrical input. That's 10:1 in actual sound pressure level power, and about "twice as loud" to the ear. Perhaps more importantly, guitar speakers are not smooth, even reproducers. They have frequency peaks and valleys that have evolved to be where guitarists like to hear them. So two different speakers will have different "voices", and also different degrees of "cutting through" the sound in a band situation.

I did a couple of PCBs that replicate part for part the preamps from the most widely known Thomas Organ Vox amps, and a few people have been building these up and installing them. One guy has done some work in adapting these in his older Royal Guardsman to work like he adapted the original (and now deceased  ) preamp. He has some really definite ideas about how to adapt the frequency response of these preamps to fit better in a band situation. I've goaded him into writing this up. We'll see if I can actually get a version of that on line.

I bought a Radio Shack SPL meter way back when. It's a really valuable tool to deciding what is really "loudest" in a listening situation. It has no psychoacoustics - I hope! - and can tell more about what's watts.

In that relation - not being used to the "feel" of  a ss guitar amp (loudness vs watts - and how hard the amp is working) I've found the Thomas Vox Limiter to be a nice addition to keep the power stage in the safe area. However it's obviously not a brick wall limiter, and from where the soft limiting start to take place there seems to be region where a sine wave gets increasingly squashed but still with increasing overall level. I take it it's the nature of the circuit – but it has to be taken into account when setting the treshold and the input level to the lim circuit. Yet it doesn't mention in the orig setup procedure (and your recent re-write in the replacement board doc). I'm guessing it has to do with the fact that in the TV amps the preamp levels are a known/incalculated factor - whereas I'm working outside of this and with maybe more drive from the "preamp", if that makes any sense at all?

It does make sense. The original write-up from Thomas about setting the limiter makes no technical sense to me. I've puzzled over it many times and
never been able to figure out what the devil they were talking about. I've done some more work on that and I hope to have it included in the Vox Owner's Safety Net, which I hope to get into publication and out of my hair. Vox advertised that thing as a safety valve, preventing overloads from killing the amps. It does that, but the missing element is where to set the power amp before adjusting the limit control. I *think* I can puzzle out what they meant, but I'm by no means certain.

For the same reason I've found adding the LED meter, with the upper LED adjusted to indicate the the power amp clipping point, a usefull addition – to show where in the safe region the amp is driven (when I haven't got the same "feel" for it as with valve amps), but also showing when peaks starts clipping. Driving the output stage into severa clipping is obviously quite noticeable.

Yep, that would be useful, all right. For some fun, twiddle it up to where the limiter is letting the power amp just clip, then dial the limiter down and listen to what happens as you turn it down. You can use it to get the same power amp "clipping" that it would otherwise do but at far smaller output levels. It's a useful adjunct to the bigger amps in getting what clipping it does, but at bedroom levels. I've often thought that the limiter control really needs to be two controls: a hidden inside one and an externalized "master clipping" or some such that lets you do the exercise above.

The original power amps are fairly fragile, with no protection built in at all. I've run the Thomas Vox power amps into LM3886-based power amps with good results. One of these gives 30W into 8 ohms pretty easily, two gets you 60 into 4 ohms, and four can do 120W into 2 ohms. The LM3886s won't go into protection if they're appropriately heat-sunk in this kind of setup, and it seems to work well.

[lion]

Thanks R.G.

I understand there's no real argument that in terms of actual power, watts is watts. It's all a PERCEPTION thing – and the fact that it takes much more than some extra watts, or even a doubling, to make a real difference in loudness, which again makes speaker sensitivity the main factor. I'll look out for a SPL meter for further experiments.

Re the limiter - I'll explore the adjustment and possibilities further. I did considered to make the treshold control front panel accessable  – your thoughts about splitting it into a two part internal/external control seems to the way to go.

I'd be very interested in the publication of your "The Vox Owner's Safety Net", as well as the possible TOV preamp freq response adaptation write-up you mention.

Erik

[R.G.]

Re the limiter - I'll explore the adjustment and possibilities further. I did considered to make the treshold control front panel accessable  – your thoughts about splitting it into a two part internal/external control seems to the way to go.

In the Vox preamp replacement boards, I changed the way the limiter adjusts from just being a 10K pot to +24V into a PNP current source driving the limiter from a much lower-power trimmer. I was after the low-power trimmer (~ 125mW instead of the 1W panel pot) and for eliminating the wires going off the preamp PCB to the limiter adjustment pot, but this actually makes controlling the limiter much more predictable. It could be turned into a real control-and-limit setup so the limiter could go on the front panel without endangering the power amp.

I'd be very interested in the publication of your "The Vox Owner's Safety Net", as well as the possible TOV preamp freq response adaptation write-up you mention.

With any luck, that thing will be available in the first part of next year. It just kept getting bigger as I went.     

The frequency response thing and another way to get lower-power limiting are coming from the same guy. He was the first (and hence very brave!! ) tester for the preamp PCBs. He's actually working on a youtube video for showing his work. I can't wait to see that myself.

[lion]

In the Vox preamp replacement boards, I changed the way the limiter adjusts from just being a 10K pot to +24V into a PNP current source driving the limiter from a much lower-power trimmer. I was after the low-power trimmer (~ 125mW instead of the 1W panel pot) and for eliminating the wires going off the preamp PCB to the limiter adjustment pot, but this actually makes controlling the limiter much more predictable. It could be turned into a real control-and-limit setup so the limiter could go on the front panel without endangering the power amp.

I can see the lower-power issue, but not sure how it affects the controlling and makes it more predictable?
I would be interested in how to turn it into a front panel control without endangering the PA – provided it'll work with the lower 15V supply I have.

Just had a look at your schem shown in the "Beatles Preamp Boards Documentation_prelim" pdf. Apart from the limiter adjustment changes I notice the E/C resistors for the PNP are a much lower value of 680R/560R opposed to 2.2/2.2k on all the original schems I have seen. Another change of yours?

Erik

[R.G.]

I can see the lower-power issue, but not sure how it affects the controlling and makes it more predictable?
I would be interested in how to turn it into a front panel control without endangering the PA – provided it'll work with the lower 15V supply I have.

It's not all that clear from the schematic, but the clipping points in that clipper are set by the amount of current through the 100R resistor you have marked with "0.06V to 1.36V". With zero current through that resistor, both diodes clip at their forward voltage. When you run current through the resistor, it raises the cathode of the "top side" diode by the resistor voltage. If we assume that the diodes clip at a fixed 0.7V, then zero current in the 100R means that clipping starts at ±0.7V. As you raise the voltage on the 100R, the voltage gets up to your 1.36V at the resistor, and the signal on the anode of that diode has to be 0.7 to 1.36V to clip, or about 2.06V, and the bottom diode still clips at 0.7V, so your signal must have a peak to peak of 2.76V to clip. The 47K resistors working with the series capacitor to the diodes force this to be symmetrical about 0 for the signal voltage in the long term, but short term signals will see asymmetrical clipping as the signal level moves the DC voltage on the series cap in response to the clippers clamping to either ground or the 100R voltage. It centers itself, and you can actually hear this in operation and see it in simulation.

So the key thing is the current in that 100R resistor. It's set by a series 1K resistor to keep it from going too big and the 22K pot plus the 1K to keep it from going too small. Changing the pot to a PNP with an emitter resistor makes the current in the PNP be almost purely a consequence of the PNP's base voltage. The current is now the PNP's emitter resistor divided by the base voltage minus one Vbe for the PNP. Tolerance of the 1K and pot are removed from the calculation, as is any offset in current from the contact resistance of the pot's wiper. A slightly more advanced version would use two PNPs in a current mirror setup, and pull current from the mirror input for "reflection" into the 100R. This current can then be made the sum of two currents, one a minumum through the clipper, and the other an "adder". You start getting control and predictability.

Just had a look at your schem shown in the "Beatles Preamp Boards Documentation_prelim" pdf. Apart from the limiter adjustment changes I notice the E/C resistors for the PNP are a much lower value of 680R/560R opposed to 2.2/2.2k on all the original schems I have seen. Another change of yours?

Not really. Thomas Organ issued a number of field update bulletins that their field techs were supposed to do to amps that came in for service. The updates amounted to the changes they made in factory production as they fixed any nagging little problems. That resistor change is from a field service bulletin. The preamp PCBs have the latest field notes that I know of applied. The technical supplements per amplifier model that go with the Vox Owner's Safety Net also include any known field updates.

I made very few changes. One was voltage regulators; another was the control setting method but not the operation of the limiters; another was using the V1143-style JFET switching instead of relay switching for MRB on the V1141 amps; addition of CMOS signal switching as an option instead of the rotary switch in reverb assignment (it can be wired either way); and on-PCB mini-relay for switching distortion in the V1141 line.

[Hatredman]

I've often read the comment that "valve watts are louder than SS watts".
From what I've tried so far I have to agree – and it seems not just a subtle difference. The HH S130 I use is suppossedly 130w @ 4 ohms (A quick measurement showed 25 volts output at the point of clipping across a 8 ohm dummy load  – that's > 80w, right?) Even so the SS rig running through a cab of Celestion Greenbacks (not that unsimilar to the AC30's Blue's - is fighting to keep up with my AC30. Interesting.

I think this is already said, but you DO have to run both amps through the same cabinets and speaker, or the A/B comparison is useless.

Aural Hamonics (or whatever you call it, and in fact any harmonycs) have a psychoacoustic effect on us that makes us perceive the sound as louder. But it's a brain thing, and you have to hear it to have this impression. Even the cleanest 70's Fender Silverfaces will add a little bit of distortion to the sound, that will be perceived by us as louder (als also "rounder", "sweeter") than a Solid State counerpart, as the SSs (if well engineered) will not add any hearable harmonic distortion.

If you compare a 10W tube amp with a 20W SS amp, with no other intruments, just the amps and the same guitar, all gain and volumes way up but well before crunchness or clipping, maybe the 10W tube will sound louder, driving the same cabinet+speakers, even if the 20W SS is "delivering more Watts". But in a band situation, with an ogre drummer, PA, foldback etc, the few more watts of the SS will show, and the apparently "louder" tube amp could even get covered and not be heard.

But you have to take out all the other variables of the equation, so you MUST run both amps through the same cabinet+speaker. Do this, and you'll see that even without the SPL meter your perception of the difference in loudness will be more subtle.

[lion]

R.G.
Clever little circuit that limiter, looks so simple at first impression, but there's clearly more to it. I get the overall function, but must confess I'm struggling to fully understand the finer parts of your explanation.

For the time being I think I'll keep the circuit as is – at least till I've played it and experimented some more.

Henry
Thanks for your input. I understand and agree on most you said. If I wasn't so tied up right now I would have made a few comments, but maybe another time.

You bring up another aspect - the "cut-through factor". Another issue to explore further.

Erik