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DC Coupled Cathode Follower Questions
DC Coupled Cathode Follower Questions
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Old 25th March 2016, 07:04 PM   #1
steeledriver is offline steeledriver  Canada
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Default DC Coupled Cathode Follower Questions

After reading Merlin's post on the sound of DC coupled cathode followers when they're set up to steal current from the previous stage, I'd like to toy with them in my next amp, which uses a solid state tonestack. Does anyone have anecdotal evidence to support his praise for their effect on the sound in an amp?

I'm considering using them in two places in the amp - once right at the input before the tonestack, buffering the tonestack, and once right after, immediately before the PI (see attached). Is it a good idea to generate that kind of distortion BEFORE the tonestack? Or even a good idea to stack that kind of distortion in an amp twice? I'd like to hear it with an inverted signal (compressing both peaks) and a non-inverted signal (compressing positive peaks twice) for comparison.

Any experiences?
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Old 25th March 2016, 07:30 PM   #2
MrCurwen is offline MrCurwen  Finland
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I can see this is an instrument amp, not a HIFI amp. Wrong forum section.

Anyway; what kind of tone results are you looking for? Those bootstrapping and buffering things make the amp better able to drive the tonestack. This is not necessarily a good thing, unless you're looking for a very clean, dryish sound.

On the other hand, a better drive capability could make an amp less sluggish and have more snap.
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Old 25th March 2016, 07:46 PM   #3
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by steeledriver View Post
Is it a good idea to generate that kind of distortion BEFORE the tonestack?
Absolutely. Applying EQ to a pre-distorted signal has greater effect than doing it before distorting the signal.

Quote:
Or even a good idea to stack that kind of distortion in an amp twice?
Yes, several high-octane commercial amps do this. However, with only one stage of gain before the stack, you won't get a huge amount of distortion unless you have active pickups.
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Old 25th March 2016, 08:35 PM   #4
soulmerchant is offline soulmerchant  Switzerland
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The bootstrapped follower certainly does work. Pretty darn low distortion values for a simple all-tube circuit and the frequency response can be made very flat too. All of the sudden the low-reputation 12ax7 will do surprising things...

Tips:

1. D1 is usually not so necessary if you have a slow start-up... but if you use solid state rectification and are concerned about protecting the valve, a neon bulb works very well.
2. It's far better to use higher B+ and then use greater anode load (at least 2x 100k ohm) on the differential stage.
3. You will need a far larger C1 if you want to get frequency response super linear. These days I am using 2uF or sometimes a bit higher.
4. I think you might want to re-calculate your value for C2 as well...

Of course if you are looking for a distorting circuit then maybe its not a wise choice...

Ian

Last edited by soulmerchant; 25th March 2016 at 08:43 PM.
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Old 26th March 2016, 04:23 AM   #5
steeledriver is offline steeledriver  Canada
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Quote:
Originally Posted by Merlinb View Post
Absolutely. Applying EQ to a pre-distorted signal has greater effect than doing it before distorting the signal.


Yes, several high-octane commercial amps do this. However, with only one stage of gain before the stack, you won't get a huge amount of distortion unless you have active pickups.
Hi Merlin, thanks for checking in. I own your book, and it was a godsend to me two years ago when I picked up this hobby. It's great to be able to clarify ideas with the person who engendered them.

If an overdrive channel were added on a switch after the first gain stage in the form of an extra gain stage and a second DC coupled cathode follower, the overdriven signal would be inverted and have the cathode follower distortion applied to the other side of the wave. With appropriate attenuation so that the op-amp tonestack doesn't clip, do you think that would make a desirable sound? I think that would lend itself to removing a lot of high harmonics from the signal.
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Old 26th March 2016, 04:36 AM   #6
steeledriver is offline steeledriver  Canada
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Quote:
Originally Posted by soulmerchant View Post
The bootstrapped follower certainly does work. Pretty darn low distortion values for a simple all-tube circuit and the frequency response can be made very flat too. All of the sudden the low-reputation 12ax7 will do surprising things...

Tips:

1. D1 is usually not so necessary if you have a slow start-up... but if you use solid state rectification and are concerned about protecting the valve, a neon bulb works very well.
2. It's far better to use higher B+ and then use greater anode load (at least 2x 100k ohm) on the differential stage.
3. You will need a far larger C1 if you want to get frequency response super linear. These days I am using 2uF or sometimes a bit higher.
4. I think you might want to re-calculate your value for C2 as well...

Of course if you are looking for a distorting circuit then maybe its not a wise choice...

Ian
Hi Ian - I posted this in the wrong forum. I'm building a guitar amplifier, so distortion is somewhat the name of the game.

Thanks for the great advice. Indeed, I would prefer a slow start up to avoid a transient spike that might damage the input of the op-amps. My LTSpice simulations show that the positive input only spikes to about 25V, but that's just a simulation and I'd prefer to play it safe. On that note, can you recommend a simple yet effective soft-start circuit? I was thinking of just using a varistor.

I chose 0.01uF to attenuate the bass around 60Hz in the amp. The tone controls I designed have +/- 8dB of control, so I'm worried about having too much booming bass in the amp. My previous amp is very bassy even with the Dumble-style bass control bottomed out.
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Old 26th March 2016, 07:24 AM   #7
Mooly is offline Mooly  United Kingdom
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DC Coupled Cathode Follower Questions

Moved to I&A forum.
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Old 27th March 2016, 04:44 AM   #8
steeledriver is offline steeledriver  Canada
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Thanks Mooly, my mistake.

Still unanswered, for those just joining:
- Soft start method - thermistor, varistor, or other?
- DC coupled cathode follower used twice with an inverting gain stage in between. Has it been done before? Does it work well to smooth a fizzy distortion? Or is it better to apply to a non-inverted signal?
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Old 28th March 2016, 07:42 AM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Do you have any experience with how the positive peak compression affects the sound of an otherwise clean signal?
Well, this is a form of distortion, so it's not really a clean signal anymore! But initially it's just a fattening up of the tone.

Quote:
If two gain stage/DCCF blocks were placed in series for overdrive, the overdriven signal would be inverted and have the compression applied to the other side of the wave, in addition to the initial compression on the clean signal in the first stage. With appropriate attenuation so that the op-amp tonestack doesn't clip, do you think that would make a desirable sound?
Yes. What matters is the biasing of the two gain stages, and the loading of the cathode followers. You will need to experiment with these for the best sound. Use 10k trim pots for the bias resistors to begin with, or something of that nature.

Quote:
I think that would lend itself to removing a lot of high harmonics from the signal, and my aim is for a smooth overdrive with only 2 valves.
Leave yourself the option of bootstrapping the gain stages. This will allow you to get a more crunch if you decide you need it.
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Old 29th March 2016, 10:20 PM   #10
gingertube is offline gingertube  Australia
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They say a picture is worth a 1000 words - so here is an example.
http://bmamps.com/Schematics/marshal...eSchematic.pdf
The Marshall 30th Aniv. Special.
Sheet 1 of the schematic shows the Clean Preamp (top), the Crunch Preamp (middle) and the Lead/Overdrive Preamp (bottom).
Note 3 off DC Cathode followers in the Lead/Overdrive Channel.

The V2B switchable operating point stage in the Crunch Preamp is another "iconic" circuit which shows up in many amps from many manufacturers (that is it has been copied many times).

Cheers,
Ian
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