Hello R.G. ...

[Dragonfly]

...I had a quick question for you, if you have a second. And please, keep in mind that I'm still more of a "tweaker" then a "technician"

Anyway, I came across an old op amp based tremolo circuit, and it "seemed" like all you would need to add to make a working circuit for guitar usage was adding a buffer into it, and perhaps a bit of "gain makeup" on the output. I was wondering if you could take a quick look at it and see if you spot any "obvious problems".

Thanks in advance !

AC

[R.G.]

Yep. That's the tremolo circuit right out of the pages of the old National Semicondcutor Audio Handbook. "Floobydust" chapter, last page as I remember. I have the book here somewhere.

The gotcha is in the fact that it uses diodes as a variable resistance.

It's a half-ring version of the old diode ring modulator. The LFO puts a variable amount of current into the two 1N914's, and this changes them from not conducting at all - infinite resistance - to conducting heavily, perhaps only a few ohms. The diodes are run in parallel to the LFO and in series to the signal.

However, as you're wondering about by now, diodes really do have only a 0.7V forward drop. The incremental resistance you're modulating is the resistance to signal for signals so small that they are insignificant compared to the forward drop. In general, diodes start having unacceptable distortion on signals bigger than about 25mV. So  this setup works OK for signals under 25mV, and starts distorting above that. Guitar is about 100-500mv, so you have to attenuate the incoming signal between 4:1 and 20:1 to keep it down to an acceptable distortion level.

The other thing is that the forward drop of the diodes changes from maybe 0.4V to 0.7V as you sweep the LFO, and as shown that voltage change is fed right into the output JFET buffer and amplified. The 0.1uF and 10K load on the drain of the JFET is an attempt to get rid of some of that, as the rolloff of that 0.1 and 10K is up at 159Hz. It's down 3db there, and 23db at 15.9Hz down in LFO range, so the output network is an attempt to suppress feedthrough.

I built that one back in the mid 70's when the Audio Handbook came out. I didn't like it. It both distorted and thumped.

The Thomas Organ Vox amps used a variant that actually works much better. I've though several times about putting one of these out as a project. Then someone copied my stuff without permission, or pulls snotty tricks with other people's stuff and I think - "why bother?"

The Thomas Organ stuff uses  four diodes, two strings of two in parallel. The audio is injected at one anode/cathode node and taken out the other. The anode/anode node and cathode/cathode nodes are driven out of phase by the LFO symmetrically around a bias voltage. If the diodes are well matched, the thump is cancelled out right there. In practice you get better than 20db thump suppression there, and you can highpass out another 10-15db later, so it works well. The Thomas amps use a low internal signal level anyway, down in the 50mV range, so this works out OK. The Thomas Vox amps have a good sounding tremolo.

On a side note, I went to school with and worked some projects with John Maxwell, the guy credited with a bunch of National's JFET applications notes. He was a great geek.

[Dragonfly]

Yeah, I was hoping that adding that fet buffer at the front, and the fet makeup circuit at the end would perhaps make it useful...

i have to go "digest" your post now

You never fail to teach me something i didnt know, RG...thank you !

AC

[slacker]

If you're interested I did a tremolo based off that schematic for the first FX-X competition. It's in this thread
http://www.diystompboxes.com/smfforum/index.php?topic=48815.0

It worked quite well but like RG said it's easy to make it distort and especially at higher speeds the thump was pretty bad.

[Dragonfly]

Interesting read, R.G. ... http://www.geofex.com/article_folders/voxamp/voxprot.htm ...I had never seen that on your page before !

After reading your post, now I've gone off on a "Vox schematic" tangent. I'm like that : I get interested in something, and I have to go gather as much info as possible and figure it out.

And when you say "I've though several times about putting one of these out as a project. Then someone copied my stuff without permission, or pulls snotty tricks with other people's stuff and I think - "why bother?" ...I understand completely.

Thanks again R.G. ...for the information and inspiration !

Have a great day !

AC

[Dragonfly]

If you're interested I did a tremolo based off that schematic for the first FX-X competition. It's in this thread
http://www.diystompboxes.com/smfforum/index.php?topic=48815.0

It worked quite well but like RG said it's easy to make it distort and especially at higher speeds the thump was pretty bad.

Just went and checked this out. Pretty interesting, as I have a BUNCH of 4069's sitting around here...I might do a bit of further experimentation with this one, if you dont mind.

thanks for showing me the link...

AC

[slacker]

..I might do a bit of further experimentation with this one, if you dont mind.

Yeah that's cool, see what you can do with it. I keep meaning to go back and see if I can improve it, the original was chucked together pretty quickly for the competition.

[stm]

...The Thomas Organ stuff uses  four diodes, two strings of two in parallel. The audio is injected at one anode/cathode node and taken out the other. The anode/anode node and cathode/cathode nodes are driven out of phase by the LFO symmetrically around a bias voltage. If the diodes are well matched, the thump is cancelled out right there. In practice you get better than 20db thump suppression there, and you can highpass out another 10-15db later, so it works well. The Thomas amps use a low internal signal level anyway, down in the 50mV range, so this works out OK. The Thomas Vox amps have a good sounding tremolo...

My 2c:

The four diode balanced arrangement to cancel out (at least partially) the control voltage is a clever one.  I've seen some circuits that use several diodes in series to raise the input signal allowed.  I bet using two silicon diodes in series doubles the input voltage range for a given THD level, just as three diodes would triple it.  I don't know if a red LED (with a voltage approximately three times that of a silicon device) would effectively mean thrice the input dynamic range, though.

Also, the fact that some feedthrough will always remain suggests that this type of circuit is suitable for sinusoidal modulation only, as the harmonics in a square, triangle, ramp wave would feedthrough no matter how much you filter out low frequencies.

Another aspect that would improve balance in the four diode arrangement is to sort and match the diodes with the aid of a DVM in the diode position.  I measured a good bunch of diodes the other day just for curiosity and found out that within the 1N4148's I had there were two different groupings, probably belonging to different purchases and fabrication lots.  It should be easier to match diodes than JFETs.

Additionally, it would be ideal to drive the diode network with a CURRENT source instead of a voltage source.  Why? because a current source has an infinite impedance and would produce a high impedance path for audio signals.  The current circuits use a rather high voltage with a moderate value resistor which somehow approximates a current source.

Finally, it is interesting to study some synthesizer circuits that implement voltage controlled filters and the like using atrings of diodes whose operating point is varied.

[R.G.]

Great minds run in the same ruts. I've been down some of these paths before.

I don't know if a red LED (with a voltage approximately three times that of a silicon device) would effectively mean thrice the input dynamic range, though.

The region of V-I curvature in the knee of an LED is about the same size as a silicon diodes. Not much advantage there, and lots of disadvantage in the higher voltage.

Another aspect that would improve balance in the four diode arrangement is to sort and match the diodes with the aid of a DVM in the diode position.

Better yet, get a diode array or transistor array chip intended for matched devices.

It should be easier to match diodes than JFETs.

Not really. You build up a tester, then run 'em through, sorting into bins as you go.

it would be ideal to drive the diode network with a CURRENT source instead of a voltage source.

I used a trio of current mirrors for that purpose. One master to drive the other two, a PNP on top and an NPN on bottom.

Finally, it is interesting to study some synthesizer circuits that implement voltage controlled filters and the like using atrings of diodes whose operating point is varied.

Notably the Hi-Fli and Little David/Rotovox phasers, the diode ladder filters, the Steiner filters and the old PAIA notch and wah circuits that used diodes in the T of a Twin T.

[rocket]

Please use a topic that describes what's your question is about.

Not everyboy has the time to read all postings.

[Dragonfly]

Please use a topic that describes what's your question is about.

Not everyboy has the time to read all postings.

Well, lets see...the topic specifically addresses "R.G.".

Is your name "R.G."  ?  No ? Well then why did you open this thread?

Next time please be more observant.

AC

[Auke Haarsma]

Please use a topic that describes what's your question is about.

Not everyboy has the time to read all postings.

Well, lets see...the topic specifically addresses "R.G.".

Is your name "R.G."  ?  No ? Well then why did you open this thread?

Next time please be more observant.

AC

With all respect to a great poster, that's *not* a nice reply.

Though the wording of Rocket could have been a bit more friendly, he does have a valid point. If you want to send a msg to RG only, I'd suggest you use PM or e-mail. If you wanna share the knowledge/question etc with the community (my guess) a better title could be used. Simply adding (amp-trem question) in the title would have done the trick.

Anywayz, keep up the good work !

[rocket]

I didn't mean to be unfriendly.

Sometimes it's hards get the right "feel" in a foreign language, which English is to me, especially when tying to be concise.

regards

Rocket

[Dragonfly]

Please use a topic that describes what's your question is about.

Not everyboy has the time to read all postings.

Well, lets see...the topic specifically addresses "R.G.".

Is your name "R.G."  ?  No ? Well then why did you open this thread?

Next time please be more observant.

AC

With all respect to a great poster, that's *not* a nice reply.

Though the wording of Rocket could have been a bit more friendly, he does have a valid point. If you want to send a msg to RG only, I'd suggest you use PM or e-mail. If you wanna share the knowledge/question etc with the community (my guess) a better title could be used. Simply adding (amp-trem question) in the title would have done the trick.

Anywayz, keep up the good work !

next time I'll just PM  R.G.

Sorry to have taken up huge amounts of your time by posting this thread with a simple "Hello RG", in hopes that he'd see it.

AC

[Dragonfly]

I didn't mean to be unfriendly.

Sometimes it's hards get the right "feel" in a foreign language, which English is to me, especially when tying to be concise.

regards

Rocket

Its ok.

AC

[The Tone God]

Well...silly me...I figured that RG would see this and answer...and "possibly", quite possibly, someone else might be interested...so I posted it here.

Next time I wont bother. I'll simply PM him and have the discussion in private, where others might not have the opportunity to learn from him.

Typically in forums when a thread contains someone's name it usually indicates that the thread is to be responded to only by the person named excluding others. That is somewhat of an unwritten protocol hence why some people may be thrown off. I'm sure it was unintended on your part.

As some people would find this information handy in the future it may be hard to find later without a descriptive thread title. I think all people are asking is to include a quick description of the content in the title to serve as a breadcrumb for search engine. There is room in the subject space for it. This will make the thread more easily found later on when searched for.

Andrew

[Dragonfly]

Well...silly me...I figured that RG would see this and answer...and "possibly", quite possibly, someone else might be interested...so I posted it here.

Next time I wont bother. I'll simply PM him and have the discussion in private, where others might not have the opportunity to learn from him.

Typically in forums when a thread contains someone's name it usually indicates that the thread is to be responded to only by the person named excluding others. That is somewhat of an unwritten protocol hence why some people may be thrown off. I'm sure it was unintended on your part.

As some people would find this information handy in the future it may be hard to find later without a descriptive thread title. I think all people are asking is to include a quick description of the content in the title to serve as a breadcrumb for search engine. There is room in the subject space for it. This will make the thread more easily found later on when searched for.

Andrew

The search engine "searches" the whole post, though...thus meaning that it will be "found" either way.Unless of course, you opt to "only" search thread titles. .

AC

[stm]

I think a "Hello R.G.: Question on opamp & diode tremolo..." title would've served all the purposes and probably catch more replies from other people as well.

[Dragonfly]

I think a "Hello R.G.: Question on opamp & diode tremolo..." title would've served all the purposes and probably catch more replies from other people as well.

n/p

[The Tone God]

The search engine "searches" the whole post, though...thus meaning that it will be "found" either way.Unless of course, you opt to "only" search thread titles. .

That is partial what my concern is. Alot people, I would guess, when going through search results just read thread titles. Even if they read the quoted portion of the thread that is included with the search results usually just the area around the search words are displayed which may not give an indication of the quality of all the information in the thread. This happens often when threads get hijacked or develop into other topics making searches sometimes hard to do.

IMHO this thread has some good information and I just want to see it's topic visabilty as high as possible for others to find. No biggie though.

Andrew