Running the Crack without a volume control

[Paul Joppa]

There is an optimum gain setting for each item in a signal chain, steering between overload distortion and background noise.

Back in 2007 I wrote a 5-page article on this question; it used to be on the archives/community page but I discovered this morning that the link is dead. Email me paul at bottlehead dot com and I'll send a PDF. It's a little dated but covers the theory well enough.

[CrackMeUp]

I have two pairs of tube monoblock amps - 300B PSET and 2A3 SET - and neither have in-built volume control. The signal feed goes straight to the grid of the input tube. In both cases then, the amp is running at it's full potential with no additional resistance in the signal path before the grid. That resistance is provided by the external preamp. So my thinking is that while the amp/s are running at full potential, the incoming signal is still being attenuated. In my case, by a transformer rather than resistors, which I prefer. So the amp still isn't being driven anywhere close to max at normal listening volumes. It's going top be fun to try a before and after test, but I'm pretty sure that controlling volume with an external TVC is going to yield improvements, just as, I'm sure, the BH stepped attenuators do. I don't think it matters whether the VC is in the same box as the amp or at the other end of the interconnect in the preamp if all you're using to control the volume is another stepped attenuator. Looking forward to testing the theory.

Steve

[diynewbie]

I'm  having a hard time following this thread.  You all need to to dumb it down for me.

My thinking is that both preamps and amplifiers are fixed gain devices.  Paul Joppa's point is that circuitry and components are optimized to provide the cleanest gain of a range of input voltage to output voltage.  Another way of looking at it is more or less gain would be possible, but it wouldn't be as clean.

Input voltage in the gain stage in a preamp or headphone amp is controlled by an attenuator, which reduces the source voltage to an appropriate level.

I had to look up TVC, but my understanding that it is either a volume control with a step up transformer or a varible transformer with the former being the most likely.  It is an attenuator followed by a transformer that acts as the gain stage.

So, the proposal is to remove the attenuator in the crack and replace it with an attenuator  followed by a step up transformer. (the TVC).  The attenuator in the TVC has to reduce the source voltage to remove the gain from the transformer and then reduce it further to obtain the appropriate level of voltage for the gain stage in the crack.  Is this correct?

[Paul Joppa]

TVC is usually a variable step-down transformer (or autoformer...).

[AB2KH]

Good Day All,

1 Purchase kit (this will help Doc with his salt flat racing program)

2 Assemble kit AS YOU DESIRE (If you deviate from the manual you are on your own)

3 Pass resistance tests

4 Pass voltage tests

5 Instal in your system AS YOU DESIRE

6 Turn amp ON allow it to warm up

7 Pore Bourbon

8 Select headphones

9 Select music file

10 Press PLAY, sit back and enjoy

 This is not complicated stuff

  Chris/AB2KH

 There is no cure for Audiofilianervoasa, however, thankfully, it can be effectively managed.

[CrackMeUp]

So, I've built my Crack and I'm loving it. I have the Speedball upgrade built and waiting to install after I get some listening hours on the basic unit, but my thoughts are turning back to this question I raised before I ordered the kit.

Rather than upgrade my Crack too much, I think I'm going to order a Carackatwoa and upgrade that, as according to everything I've read, an upgraded Crack will never better an upgraded C2A and I want to stick with OTL. That said, I still want to do away with a volume pot in the amp.

Listening tests in my main system, using the previously mention TVC as a stepped attenuator between source and amp has shown a night and day difference in sound quality in favour of the TVC as volume control - as one would expect given the price of a good TVC vs a carbon track potentiometer. So, I plan to change the pot on the crack and have it as a portable headphone amp, while the C2A will always be used in the main system with the TVC.

Now, thanks for your patience. The question is, will it be sufficient to just drop a 100k resister from the grid of the input tube to ground and the signal straight to the grid. That's effectively what's happening when I run the Crack-pot (lol) wide open, so my thinking is that this would be fine.

But... In many similar tube amps schematics, I see a 1k resistor in the signal path between signal and grid (part of a low pass filter?) and a 1m resister to ground, often as well as a 100k-250k pot where one is present. Can anyone tell me why this might be necessary or desirable. I understand that the 1m resistor is giving the grid of the input tube a reference to ground, presumably in the event that the tube should fail, but isn't the 100k resistor (or pot carbon track) also providing this? Why do some amps have both?

All answers and thoughts gratefully appreciated.

-- Steve

[Paul Birkeland]

You can just move the wires from the middle pot lugs to the adjacent outer pot lugs to accomplish this.

[Doc B.]

Just FYI we tested and rejected TVCs when we developed the stepped attenuator used in the TwoQuiet and our other premium kits. Not saying you won't like TVCs. Just saying it will sound different than what we decided we liked best.

[CrackMeUp]

Thanks Paul/Doc.

I understand that a simple wire change on the pot will effectively hardwire the signal to the grid, bypassing the pot. The question was more about the presence of a commonly seen setup in tube amp schematics, using a 100k pot and/or the other two resistors mentioned. I just wondered why circuits are often designed with those additional resistors and what they might do (or not) for a circuit such as this.

TVC vs stepped resistor attenuation is another matter altogether really, and I'm sure results vary greatly depending on the quality of the transformers used, as it must be with the quality and matching of resistors. It's undoubtedly cheaper to build a good resistor based attenuator than a good transformer based one. I have a very good TVC - the Music First Audio Baby Reference - and for my setup, it's the better option for me vs any other kind of volume control as it sounds great and is also my preamp for input switching.

Just curious about the use of the other resistors before the input grid on amps of similar design.

[Paul Birkeland]

But... In many similar tube amps schematics, I see a 1k resistor in the signal path between signal and grid (part of a low pass filter?) and a 1m resister to ground, often as well as a 100k-250k pot where one is present. Can anyone tell me why this might be necessary or desirable.

The 1K resistor in series with the grid is a stopper and we haven't found them to be particularly necessary with a 12AU7.  The extra grid to ground resistor keeps the tube happy if your pot fails.   You could certainly add these resistors from each grid to the center post of the 9 pin socket, but if the pot wiper fails you'll get no sound and not really a whole lot else.  In some other circuits, loss of a pot wiper can cause significant damage that's definitely worth preventing!

[Doc B.]

We used Stevens and Billington TX102s. Essentially the same TVC you are listening to.

[CrackMeUp]

The 1K resistor in series with the grid is a stopper and we haven't found them to be particularly necessary with a 12AU7.  The extra grid to ground resistor keeps the tube happy if your pot fails.   You could certainly add these resistors from each grid to the center post of the 9 pin socket, but if the pot wiper fails you'll get no sound and not really a whole lot else.  In some other circuits, loss of a pot wiper can cause significant damage that's definitely worth preventing!

Thanks very much Paul. That's just what I wanted to know. So the 1m is a kind of backup to save damage in the event of tube failure and/or pot wiper failure. Got it and much appreciated.

[CrackMeUp]

We used Stevens and Billington TX102s. Essentially the same TVC you are listening to.

Yes, much the same, but the RX63 I believe in the reference models. The main point for me in using what I have is that (apart from sounding very good indeed in my setup) iThe TVC preamp is part of the system the Crack or eventually the C2A will join, and there's no point adding more resistors to the signal path, pot or stepped resistor attenuator. Thank you for the information though.

--Steve