Placement of tone stacks

[frank754]

I've read quite a bit about this in the past week, and have seen some of the threads here as well discussing this, so I hope this is not too redundant.
Most of the hi-fi amps projects I've seen have no tone controls, and the guitar amps do have suitable ones for their purposes (FVM stacks).
For hi-fi, the Baxandall is recommended (Duncan's amp pages & others).

I've found this to be a good Baxandall schematic, perhaps it will work adequately for hi fi:



The question is exactly where to put it, in this amp perhaps right between the plate output of the first element of the 12AU7 (adding a .02 DC blocking cap), and the output of the tone stack directly into the grid of the second element of the 12AU7. Will this work?



If I were (rather) to put it in this preamp:

http://www.angelfire.com/electronic/...ain_Block.html

would it be best to put it between the two stages (just after the .022 cap), or would putting it at the end be ok?
What I also am thinking, if putting it at the end is ok, pehaps I could just build a little "tone-stack" unpowered box, with an RCA jack on each end, and put it between the preamp and amp for experimentation, if would have no power, just the 2 pots and components, can this most likely be a decent option?

[ray_moth]

Have you seen this article, also on Max Robinson's site?

Basically, the James (sometimes known as 'passive Baxandall') tone stack needs to be preceded by a low output impedance stage and followed by a high input impedance stage. Note that you cannot have a tone stack inside an NFB loop, because the NFB would be forever in conflict with it.

Your first idea of putting it after the first stage of the main amp is an unusual approach but it might work OK in this case, since there is no NFB loop involving the first stage. The relatively low plate resistance of a 12AU7 is probably suitable for feeding the tone stack, provided that you add a bypass cap across the cathode bias resistor. You will get a very high impedance looking into the concertina splitter, once you have converted the splitter to capacitor input and cathode bias in the usual way. (Note that this will not be a feasible solution if you ever decide to add global NFB back to the cathode of the first stage!). You will need to add a preamp with 12dB gain, because the James tone stack has an attenuation factor at 1kHz of 12dB. Max Robinson's preamp, as per the second schematic you posted, would be suitable.

Your second idea of putting the tone stack in the middle of that preamp is not feasible, because there is already an NFB loop between the two stages (see Rf in the schematic).

Your third idea of putting the tone stack after the preamp should work OK, because although a 12AX7 under normal no FB conditions has too high a plate resistance to feed a tone stack, the NFB used in this case should lower the OP impedance considerably. Since you're going to need the extra amplification anyway, this is probably the best solution of all. It avoids having to modify either the preamp or the main amp. Note: if you decide on this approach, do not add a cathode bypass cap to the first stage of the main amp, because that would lower its input impedance, which is not what you want.

[frank754]

Great, I really appreciate your comments, and this fills me in a great more in the picture of how the results would be. I didn't see on first glance the problems of putting it between stages of the pre-amp.
After the pre-amp would have been also my second best guess.
So based on that I think I could do well putting the full tone stack at the end of the (separate) pre-amp, hoping that myself or anyone using the design could derive some selective range enhancement from it.
Basically I like the tube hi-fi sound schema in general, but the cases I have had, for example, have been coming either from a turntable with mostly midrange and a lack of bass, as well as others coming from an old CD deck with excessive bass. The "treble", "midrange" and "bright", seem to be ok in most scenarios.
As of the first guess (across the 2 elements of the 12AU7), are you saying this is basically ok, but I need to parallel the 6800 ohm cathode resistor with with a capacitor value, in what range would be a ballpark figure?
Thanks again.
Yes, I've been to Max's site and did a bit of research here & there, but as I've been slack on the tube stuff for over 25 years (till now), it takes a bit to sink in sometimes.

[ray_moth]

are you saying this is basically ok, but I need to parallel the 6800 ohm cathode resistor with with a capacitor value, in what range would be a ballpark figure?

I think it should work OK but the unbypassed cathode would cause the 12AU7 to have an unnecessarily high OP impedance. The reason there isn't one there already is probably to provide some current feedback to lower distortion and to stabilize the gain of that stage, since it has no other NFB acting on it. It has the added benefit of avoiding having an electrolytic cap in the signal path, which many do not like because of poor sonics.

That's all well and good but current feedback also has the effect of increasing the stage's OP impedance (unlike voltage feedback, which reduces it as in Max's 2-stage preamp). High OP impedance is not what you want for feeding a tone stack. A suitable bypass cap here, with a 6.8k cathode resistor, would be 22uF @15v DC working.

A much neater solution, though, is to leave the amp circuit alone and simply put the tone stack on the front, preceded by a preamp. Incidentally, any time you're not using the tone stack, you probablt won't need the preamp either.

Are you aware of the tone stack simulator available from Duncan Amps? It includes a James type stack, among others. You can play with values, including OP impedance of the preceding stage and IP impedance of the following stage, to see the effects. The URL is here.

[frank754]

Thanks Ray, I believe I understand the entire picture now, and appreciate your time delineating this.
I'm going to bulid it as a separate box, between the pre-amp and amp, and see how that does, then do an experiment with temporarily inserting it in the amp circuit with test leads (adding the caps), and see how much attenuation of signal strength it causes.

You are correct, I probably won't need it or the pre-amp in most cases, when I feed my CD deck, or a tuner into the amp, the sound across the spectrum is already well-balanced, with a lot of gain available, enough to even overload my speakers if I crank it up too high.

This entire question was brought up due to my turntable being the exception, it needs the pre-amp to even drive the amp to a good audible volume to begin with, and even then it favors midrange, not much bass at all, so that's where I figured I could try some of this.
In fact I may build a second preamp and use it in series to see what this does, as a further consideration. So basically I have all the info I need thanks to your help.

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For a bit of extra weirdness, I was playing with the turntable & preamp hooked up into that new amp I finished (per that schematic above, pic below), and first of all needed to move all the power cords away from the signal wires, as initially they caused some induced hum in the chain, also had to connect the turntable ground wire to the preamp or I wouldn't get much sound at all.
The weird part is that I had to elevate the speaker wire a few feet off the floor using a string, or otherwise the volume was attenuated, but would get louder when I touched the preamp chassis, elevating the speaker wire cured this.
Also, the pre-amp input gain pot had a dead spot at about 2/3 of the way up (audio taper pot), and had a "click" where the circuit would go into much clearer sounding mode after that, up to the max.
When I set it at exactly that spot, I could get it to click just by approaching the chassis with my hands.
The thing that solved all of that was to ground the pre-amp chassis to a pipe. As built, the pre-amp only has the signal ground connected to the chassis (as well as the CT of the transformer secondary). The AC goes directly into the transformer, and the 6.3v non center-tapped filament secondary is elevated above ground on both sides, with a cap as well. I guess I should put in a new 3-wire power cord to ground the chassis to the wall ground to eliminate this, though I didn't notice this before with 2 other amps I fed it into.
I guess at such low signal levels, everything becomes a factor in the "field" surrounding the pre-amp.

My wife's guess is that it's just some static, being so dry lately and the fact that I was doing this on the rug, I've been feeling some static just petting the cats lately...



(The pre-amp has two 6AV6's instead of a 12AX7)

[ray_moth]

This might be a silly question and please excuse me if it is, but does your turntable have RIAA equalization? If not, then the sound you get from records will be severely cut in the bass and boosted in the treble. There are plenty of RIAA equalization preamps on the market and available as kits or DIY schematics, if you need one.

[frank754]

It's an old Technics SL-QD22, which I've probably had for almost 25 years. Quartz, direct drive, it was rated highly and was a good $125 when new back then. Actually I had gotten that to replace another older one I bought in '77 but had sort of failed too early.
I have one of my wife's turnatbales stashed in the attic, but it's probably too much newer & not worth restoring, haven't event looked at it yet.

[ray_moth]

Fine, let us know how you go.

[frank754]

I finally got a chance to build one of these little babies today, and put it between the preamp & amp. It works great, better than expected, there's a lot of bass/treble range adjustment to satisfy almost anyone.
It's in a little 2 1/2" x 5" plastic box with metal top that they have at Radio Shack.
The circuit is the one above, except that I had to put a 150k & 33k resistor in series to get close to the 180k. The brown spot over the second pot is the 330pf mica capacitor, and I used a .022 DC blocker at the input jack. There's another cap hidden under the blue one as well.