Thomas Vox limiter build - help!

[Erik Lion]

I've decided to build the Thomas Vox limiter for use in a DIY bass amp project (aiming for a 60's sound), and I was hoping that some of the resident expert can help me with some problems.
I use this schematic: http://img.photobucket.com/albums/v165/rickydon/voxtmixlmt.gif

I've already put it together on perfboard - but somethings is wrong (could be me though, happende before), and I would really appreciate some help. I've used 2N3903/06 and 1N4148 diodes. My breadboarded circuit seems to be 'dead' - and somehow is also playing tricks on the PSU. I'm not sure how to go from here - but I thinks a few answers could be at start:
1) Has anybody succesfully build the circuit from the schem - or can otherwise verify that it has no errors, and  should be working as shown?
2) The schem specifies two voltage supplies - 18V and 25V. My preamp runs on 30V - I figured that 5V increase would be OK since this voltage goes to the 10k trim, and I've used a voltage divider 780k/270k to make a 18V tap with a cap to ground. Would that be OK - or should the to voltages be completely seperated?
3) I'm told that part of the schem (first gainstage) is actually the last remains of the mixer circuit in the original Thomas Vox - if that is correct which components could be left out, leaving the limiter only?

Erik

[RDV]

That circuit is a edit by me where I removed the other two channels. I've not tried the circuit as drawn. Our only hope is that R.G. will chime in and say whether I screwed up the redraw or what.

RDV

[johngreene]

The circuit should work ok as drawn.



--john

[RDV]

My question is: Does the limiter add any gain on it's own, or is it just unity gain and then down?

RDV

[johngreene]

My question is: Does the limiter add any gain on it's own, or is it just unity gain and then down?

RDV

It adds gain. The first stage has a gain of about 50 and the next a gain of about 2. The simulation is using a 50mV peak input signal.

--john

[puretube]

John: was that a sine at the input?

[johngreene]

yes.

[R.G.]

The transistor at the front of the thing is actually one of three identical transistor circuits in the main amp. The three collectors are common.

The signal levels up to the limiter are quite low - lower than actual guitar signals. This is one reason the TV amps have a reputation for being noisy - they don't immediately get the signal up out of the noise-floor mud.

[RDV]

Gee, I guess that explains my low output. I only built half the preamp. I'm a very lazy man. I guess I'll try to finish it this weekend since I don't have a gig. I'm running the preamp at around 27v, so I guess I'll need to regulate that down to around 18v for the collectors in the limiter. Does the limiter level control sort of serve as a master volume?

Thanks John and R.G.

RDV

[R.G.]

Does the limiter level control sort of serve as a master volume?

No, it doesn't. It protects the power amp.

The power amp in the old TV amps is decidedly primitive. I believe that it could be killed by overdriving it enough in a high temperature situation. My personal opinion is that the limiter is there to make sure you cannot -ever- overdrive the power amp. The limiter catches you first.

It's a theory.

[Erik Lion]

Thanks for verifying the circuit - and John for the simulation/plots.

I don't need Q1 as I already have 2-4V clean signal out of my preamp. For this reason I've omitted the first stage - starting the circuit with a 10 mf  cap to C4 and the base of Q2.

Given the parts count and the minimal wiring, even I should be able to get it right - but something is WRONG - and I can't figure out what I've done wrong and what is happening.


I have 30V and 17V for the V1 and V2 supplies.
The 17V source is a tap from the 30V source using a 330/390k voltage divider.
Is this OK?

When I turn the trimpot R14 from min to max the main 30V supply is pulled down to 20V (also feeding the preamp). The 17V tap goes down to 11v.


I have the following voltage readings:

Q2 - 2N3903:
C: 17V (11V with R14 at max)
B: 0.25V (0.20V)
E: 0.0V (0V)

Q3 - 2N3906:
E: 17V (11V)
B: 17V (11V)
C: 0.0V (0V)

At the junction of C1, D1, D2, R9, R11: 0.13v (0.8V)

After R13 where connecting to D2, R9, R10: 0.25v (1.6V)

I've already replaced Q2+Q3 once during my debugging - I'll can do it again, but I'd like to have a comment on the situation first.

Erik

[johngreene]

I have 30V and 17V for the V1 and V2 supplies.
The 17V source is a tap from the 30V source using a 330/390k voltage divider.
Is this OK?

Nope, the 17V supply is going to want to draw about 2.5 mA of bias current and you are not going to get that through the 330/390K divider! You would be better off connecting the base of an emitter follower to the 17V to give you the current drive.

When I turn the trimpot R14 from min to max the main 30V supply is pulled down to 20V (also feeding the preamp). The 17V tap goes down to 11v.

Your 30 volt supply seems to have very little available current. If the pot is at min, the 30V supply is grounded through 1.1K So it better be able to provide 27mA just for that part of the circuit alone.

I have the following voltage readings:

Q2 - 2N3903:
C: 17V (11V with R14 at max)
B: 0.25V (0.20V)
E: 0.0V (0V)

Q3 - 2N3906:
E: 17V (11V)
B: 17V (11V)
C: 0.0V (0V)

More trouble here. I'd expect to see the following: (ignoring the sag problem, i.e. R14 is at max resistance)
Q2
C: 14.17V
B: 2V
E: 1.4V

Q3
E: 14.83V
B: 14.17V
C: 2.16V

So I'd say there's a problem with the Biasing of Q2.

If you've eliminated the preamp amplifier, how are you biasing Q2? In the schematic it gets its bias from the collector of Q1.

--john

[Erik Lion]

Your 30 volt supply seems to have very little available current.

Something is really wrong here I guess.
The 30v supply is very basic, but I it should be able to deliver the current:
Transformer (24V/630 mA!!!), set up just like in a plain old tube amp - rect. diode followed by a double RC filter (100 ohms/470mF and 470 ohms/1000mF). From the start it was only meant to drive the preamp - for the bass amp project, utilizing only 4 JFET stages (no heavy current draw here. right) - which it does nicely. The idea about the TV limiter circuit came in later calling for an additional 17V tap.

You would be better off connecting the base of an emitter follower to the 17V to give you the current drive.

How exactly could I wire up a EF to get the proper 17V/current - or otherswise mod the supply?

....how are you biasing Q2? In the schematic it gets its bias from the collector of Q1.

When Q2 gets it's bias through the Q1 collector I'm obviously missing something with the 1. stage totally eliminated. My ignorance really shows here - but how can I bias Q2 without the first gainstage?

A drawning says more than a 1000 words

Thanks for the help so far, it's really appreciated.

Erik

[johngreene]

OK, here's your drawing.....

I have the input set at 1V peak. So you need some serious input signal to make this work.

--john

[Erik Lion]

John - thanks for posting this, and for taking the time to help me out.
What do you think of my supply/sag problem - do I really need a stiffer/better filtering or regulation?

Erik

[johngreene]

I think your problem is with the resistors you have in series. You'd be better off with a regulator. You have 570 ohms in series with the supply line so for every 1mA of current draw, you will drop 1mA*570 = .57 Volts. So, I estimated you would need at least 27 mA before and that would cause 15.4 volts of drop across those resistors!

--john