[DESIGN] The Colorbox - Redoux

[Consul]

I can't remember if Bill Yacey and I had started on this thing back at the old forums or not. Either way, I still have the rev.3 schematic, with no part values, here:

http://studioconsul.net/techtalk/colorbox/colorbox_rev3.gif

Anyone interested in continuing to take a crack at this? The idea is to build a unity-gain amp with a couple of tube stages using zero feedback. Of course, you could set the attenuators slightly differently to get some increase or reduction of gain of the original signal.

For a power supply, I figure adapting PRR's 100-volt supply for his vari-mu would be workable, though perhaps a higher voltage be advisable.

So how about it?

 

[NewYorkDave]

How about one of these with a second 100K pot on the grid of V1A?
http://electronicdave.myhosting.net/miscimages/lineampE.gif

Stepped attenuators are kind of pointless in a no-feedback circuit since the gain of the tubes is not fixed. The clicks feel nice but the panel markings may or may not correspond to reality.

 

[PRR]

That's doing it the hard way. Any change of distortion also changes gain.

For a single stage, just change the supply voltage. For normal studio levels, with B+ of 400V, it will be dead-clean, with B+ of 40V, it will be dirtier than dirt, and you can find the just-right setting in between. Gain will change slightly with B+, but not as much as distortion.

For a 2-stage amp, which you need to get a mix of even and odd harmonics, it gets trickier.

 

[Consul]

Well, my original idea was to put an attenuator at the beginning of the circuit, then control the make-up gain on a tube stage. Add 2 pieces of iron, and you probably would get a lot of color.

So, just build a unity-gain stage, and put a variac on the B+ input? Sounds easy enough.

 

[NewYorkDave]

Good luck building a unity-gain stage with zero feedback...

 

[Consul]

Heh... Okay, we're talking about two different approaches here, and only one of them has mentioned using no feedback.

I think I like PRR's idea the best so far. Now, to find the best way to vary B+ within the circuit.

Now, here's a question:

I was told by Bill that using feedback would clean up the sound of the tubes, thus making the circuit less colored. Assuming that's true, what design decisions for color do you want to make in a circuit with negative feedback? Or does it really matter?

 

[NewYorkDave]

Well, if you just want to build a distortion box and you don't need fixed gain, then don't worry about feedback.

One of the cheaper/easier ways to vary your B+ is to use separate plate and filament transformers and use a small variac on the primary of the plate transformer. If you hunt around at hamfests, you can find little 1.5-amp models. The very cheapest way would be a wirewound pot in series with B+, but that's not a good idea unless a)the pot shaft is nonconductive and you'll always be well-insulated from any potential shock, and b)the plate current required is very low. The 12AU7 will draw a fair amount of plate current under most typical conditions. You could also use a suitably-insulated pot wired as a voltage divider driving the base of a series-pass transistor of suitable voltage rating.

 

[Consul]

This could be interesting... How many audio devices have their primary control at the power supply? :green:

So, variac plus a one-stage unity-gain tube amp. Sounds simple enough. Maybe I'll even have time to draw it up before I leave for Michigan.

 

[NewYorkDave]

Unity gain implies feedback, which tends to cancel distortion, so if you want distortion, ditch the "unity gain stage" idea. You can always put a pot or attenuator at the output--or use a stepdown xfmr with a suitable ratio--to make the effective overall insertion gain equal to unity.

 

[clintrubber]

to vary your B+

Or perhaps a switch providing a few different B+ values ?

A drawing would be better, but let's try:

R_a of fixed value and decoupled to ground. Connect upper side of R_a to the supply by a string of additional resistors which can be more or less shorted by the N-position switch.

Peter

 

[NewYorkDave]

Although I don't really trust simulators, here's something I just whipped up that could be interesting

With a 1V peak input, the THD (mostly 2H) varies from 0.7% to a whopping 22% as you vary B+ from 100V down to 10V. The net insertion gain (with output transformer) only varies from 5dB to 3dB.

Up around 250V, it's clean (0.4%THD) and probably uninteresting for our purposes, plus the plate current is rather high at that voltage, making it a challenge to vary the B+. So a 10V to 100V B+ gives the most interesting range of effects with greatest ease of varying the plate voltage.

If you remove C1, net gain drops to unity, and distortion decreases a bit.

Note that the output transformer is wired to reverse phase so that the output is in phase with the input at the grid of V1A.

 

[PRR]

> Good luck building a unity-gain stage with zero feedback...

Gain-stage with resistive divider. In my plan below, 12AU7 with 100K plate resistor, the gain will be about 14. Tap the 100K plate resistor as about 93K and 7K, the gain at the tap point will be about unity.

No, it won't be exactly unity-gain for all supply voltages and tube replacements. But a good solid no-feedback triode will be stable +/-20%, 2dB, good enough for rock-n-roll, especially for a box that has to be tuned by ear. Consider tube guitar amps, which typically have nearly no feedback in preamp stages, but have reasonably constant gain from day to day. Whatever gain they have, you can un-do with a pad, and get reasonably unity-gain day to day.

When working in the "colored" range, "unity gain" becomes a fishy concept. The voltage gain won't change much even with large distortion, but the apparent loudness will rise with THD.

We only need to be "unity" so that input and output levels are usable: a Line In into the color box gives a Line Out signal, without radical level shift or padding needed. +/-6dB "error" is fine, and +/-10dB acceptable.

> put a variac on the B+ input?

Way over-kill.

You don't need Power to get distortion. Work something like a 12AU7 with 100K plate resistors. At 300V supply it only needs maybe 1.5mA of current. If you can find a 100K 2 Watt pot, that could vary the B+ from 0V to 300V without overheating. Do two stages of that, so you can get 2nd or 3rd harmonic as you please. Then use a final stage working at fixed high B+ and current (or a chip-buffer) to give a strong output signal.

You can also get a wide range of distortion level at fixed B+ by messing with grid or cathode bias,and this needs even less control power.

 

[NewYorkDave]

After musing aloud about the small variac, I figured out the (rather low) power requirements and also started thinking about using a pot instead--although, as you can see from the sketch, I decided to do it at low voltage.

back-of-envelope calculations tell me that this should be just the ticket to power two channels of the circuit I posted earlier. Then again, it's late and I'm tired. Check it over for yo'selves.

 

[Consul]

Wow, thank you all for your insights! I'm learning a lot from it.

Viitalahde, the idea in my mind is to make something that will "warm up" some digital synths I have in my stable, which are a Kawai K4r, a Kawai XD5, and a Yamaha TX81Z. All three are synths I love to use (especially the K4r), but they do sound somewhat thin and uninteresting sometimes.

Think along the lines of that stupid marketing tube in the new Korg synths, but better. :green: