Hybrid Amp Build Direction/Recomendation

[Diyengineer]

Hello,

I'm planning to build my next amp. I'm looking to do a pair of purchased Class D power modules with a tube input/buffer stage in one chassis as a stereo power amp that would require signal and volume control from an upstream preamplifier.

This amp in my case would receive signal from my preamp I built. It is an SRPP wired ECC802S with a gain of about 7. But I would like the amplifier to work well with commercially available preamplifiers.

My question is what would be a good tube buffer stage design for the pair of ICE Power 700AS1's I plan to use. I read the datasheet but am unsure of what kind of gain I need to obtain full output of the modules. Possibly a simple cathode follower would work even? I don't know?
The Ice Power module has an input impedance of 8K Ohms minimum.

Ice Power module datasheet is attached.

Thanks in advance

 

[rayma]

An 8k load is difficult for tubes. The output capacitor would need to be 10uF
(unless it's in a feedback loop), and the output impedance should be 2k or less.
You likely have enough gain already, around 44dB total for line sources.

 

[Diyengineer]

Thanks

I’m assuming the 10uf cap would be the coupling cap from the tube to the class d module. What other specifics should I look for in a circuit to build I.e. cathode follower or something different?

 

[wfmali]

For 8k, a cathode follower following your SRPP preamp should do. For most modern sources, you do not require a preamp anyways.
Marcus

 

[rayma]

I’m assuming the 10uf cap would be the coupling cap from the tube to the class d module.
What other specifics should I look for

Yes, the output capacitor. A warm up/turn off muting relay at the output would protect the power amplifier's input
from HV surges due to the large coupling capacitor.

Low small signal output impedance (under 2k) is necessary for low distortion into 8k loads. Also enough drive current
must be available. For full power, the amp needs around 2.3Vrms input, or about 3.2V peak. Into 8k, that is 0.4 mA
peak output current. The tube then should have a DC bias current of several times that, more than 2mA.
Look at a medium mu triode follower like 6SN7, 12AU7, etc.

 

[Diyengineer]

how did you calculate the size of the 10uf coupling cap? Sorry, I'm a novice, trying to learn.

Thanks

 

[Diyengineer]

Thanks,

I know there a few types of different cathode follower variants. Is there a certain one I should choose and possibly a schematic to reference?

Thanks

 

[rayma]

how did you calculate the size of the 10uf coupling cap?

You want a 2Hz LF corner, and then 2Hz = 1 / ( 2Pi x Rx C )

or C = 1 / ( 2Pi x R x 2Hz )

or C = 1 / ( 2Pi x 8000 x 2Hz ) = 0.00001 = 10uF

(if the capacitor is not within a nfb loop)

 

[Diyengineer]

I seen there are a few different types of cathode follower circuits to choose from. Would you be able to recommend a certain type of cathode follower circuit to use, a reference schematic would also be a huge help if you had one available.

Thanks again!

 

[Pano]

Two threads merged. It is not allowed to start 2 threads on the same subject. Please read the rules section.

 

[rayma]

Would you be able to recommend a certain type of cathode follower circuit

Can you post the line amplifier circuit?

 

[Diyengineer]

Attached is the schematic of my preamp. The only change I made is I used 2uf output coupling caps.

 

[rayma]

Attached is the schematic of my preamp. The only change I made is I used 2uf output coupling caps.

To start, add an output muting relay to make sure the amplifier isn't damaged by start up/power down HV transients.
The turn on delay should be 20-30 seconds, and the turn off should be immediate. Use a NC shorting relay to ground.

The next thing to add is a larger output C, try at least another 2uF in parallel with each, and see how you like it.
The small signal output impedance of this circuit is around 2.7k, so not too bad. It just may work well enough
like this, without adding more tubes.

 

[Diyengineer]

I would really like to add a cathode follower input buffer in the same chassis as the ice power modules though rather than changing my existing preamplifier. My goal is to have my new amp work with most preamps both diy and commercially available.

 

[rayma]

I would really like to add a cathode follower input buffer in the same chassis as the ice power modules

In that case I'd use an op amp buffer, with input muting to protect it from tube preamps.
You'd need an entire new HV supply in the box with the amp to use a tube buffer.

 

[Diyengineer]

Yes, I planned on making an entire separate power supply for the tube buffer. I would really like to stick with tubes for a hybrid design. I’m just having trouble knowing how to design the cathode follower circuit for this application with the Ice Power modules.

 

[rayma]

Yes, I planned on making an entire separate power supply for the tube buffer.
I would really like to stick with tubes for a hybrid design.

Start here:
The Valve Wizard -Cathode Follower

 

[Diyengineer]

That was one of the designs I was looking at. I’ll put a 10uf coupling cap from there to the modules with an input mute relay in between.

Thanks!

 

[rayma]

I’ll put a 10uf coupling cap from there to the modules with an input mute relay in between.

Should work.

 

[Diyengineer]

To start, add an output muting relay to make sure the amplifier isn't damaged by start up/power down HV transients.
The turn on delay should be 20-30 seconds, and the turn off should be immediate. Use a NC shorting relay to ground.

This would mean that you have a relay normally short the signal+ input of the class D module to signal ground, and when a timer times out after 20s or so it disconnects the short to ground, correct?

Is it possible to have the signal relay just disconnect/ break the signal+ wire, or is it preferable to have it short to ground.