Van Scoyoc SRPP Amp - GvT - diyAudio
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Old 3rd February 2004, 10:26 AM   #1
GvTT is offline GvTT  Germany
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Default Van Scoyoc SRPP Amp - GvT

Hi,

having been working quite some time on my new amp project I would like some opinions and ideas on what I came up with. I think this diy-forum will have plenty of experience.

Although I am using a symmetric input signal I have picked the van scoyoc schematic. This gives me the chance to use a non-symmetric signal as input.

This amp is going to be used as a preamplifier, having a maximal input of +/- 1 V and a requested output of +/- 5V.

The output connects to an active filter. there a 13K resistor connects to ground. you can see this one at the output.

To get a more stable output signal I added a tube so the output forms the SRPP scheme. I used +/- 150 to get an output at 0V.

I am really curious for your answers.

Greetings to all
Georg
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Old 3rd February 2004, 01:42 PM   #2
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Just two quick comments:

1. you can change your input caps to more like 100n and still have excellent low frequency response

2. your OCL zeroing scheme may only partly work. the way you have it, you're changing both outputs at the same time. you will need this control. but, if there are tube mismatchings then you might need to adjust one side separately from the other to get them both at DC zero
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Old 3rd February 2004, 01:43 PM   #3
SY is offline SY  United States
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I see that you've RC coupled the input, but direct coupled the output. How are you going to control offset?

edit: I see R8 had the same thought.
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Old 3rd February 2004, 02:13 PM   #4
GvTT is offline GvTT  Germany
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Default Offset correction

Hi,

The offset-dismatch between the tubes and the common offset are a concern. I don't know how much of a drift during heat-up to expect.

I could imagine using an op-amp and condensator to generate a mean-signal-value and feed this information into the active-filter to compensate the drift. But this means that I have to have a drift of aabout +/- 5V maximum.

I don't know if shifting the whole power-supply with a mosfet for example would be a good idea.

But since I am running it as an active symmetric system (bridged) I am more concerned about the offset between the two chanals.

And if everything fails I can go to a +300/0 V supply and ad the coupling condensators at the output.

I will have to give this offset some more thought.

Thanks for the hints
Georg
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Old 3rd February 2004, 03:34 PM   #5
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If you're more concerned about the balance and not the absolute value of DC at the output, then you can do this. replace your 2K rheostat with the following:
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Old 3rd February 2004, 03:37 PM   #6
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Forgot.

I'm not sure how your power amps (or whatever is being driven) will like significant DC current into their input stages.

Also, if you go single polarity supply, then you're input which is also coupled to the SRPP cathode legs will see a high B+.
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Old 4th February 2004, 07:33 AM   #7
GvTT is offline GvTT  Germany
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Smile Offset correction

Hi,

I tried to work in an active offset correction. But this became so complicated, that I now believe, shifting this whole circuit to +300/0V and using a coupling condensator at the exits will be the best choice.

If you connect the plates of the bottom tubes to ground (in the schematic using symmetric power-supply) the bandwidth is significantly improved, the THD increases to (at least according to the simmulation).

Greetings
Georg
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Old 4th February 2004, 10:15 AM   #8
EC8010 is offline EC8010  United Kingdom
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Default Re: Offset correction

Quote:
Originally posted by GvTT
If you connect the plates of the bottom tubes to ground (in the schematic using symmetric power-supply) the bandwidth is significantly improved, the THD increases to (at least according to the simmulation).
So shouldn't you ask yourself why a change in absolute DC voltages (but no change in currents) changes bandwidth and distortion?
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