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SRPP Preamp hissing
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Author:  RevM [ 19 Jun 2020, 13:32 ]
Post subject:  SRPP Preamp hissing

Hi all,

Just built my second SRPP preamp. They really do sound fantastic!
However, I have a HISS (like white noise) on both channels equally. I have swapped the power supply resistors, no difference. I am regulating LT with fixed 6v regulator, and the HT with an LR8.

Where should I look next? Surely it must the power supply section since it is the same on both channels?

Author:  gofar99 [ 19 Jun 2020, 14:17 ]
Post subject:  Re: SRPP Preamp hissing

Hi, Well that depends on a number of factors. There is a minimal level of noise inherent in tubes. Usually it manifests itself as hiss. You didn't specify when the hiss is audible. So some more information is needed. If you built one of the ones like the Forewatt then the residual hiss is at the -80db or lower level. Really quiet. So let us know what it is feeding and when the hiss is noticeable.

Good listening

Author:  RevM [ 20 Jun 2020, 03:39 ]
Post subject:  Re: SRPP Preamp hissing

Hi Bruce,

Thanks for your swift reply.
The circuit I have used is your ForeWatt design with the addition of an LR8 regulator for the B+. I have added the LR8 because I am using a UK made mains transformer that puts out 190-0-190 so, after smoothing, I need to reduce that a bit. Rectification is via two diodes (rated at 600v) and the centre tap. I have added a fuse in HT and LT just for additional protection. I've also added a thermistor after the diodes, before the fuse, in the HT to attenuate the inrush current. Power supply resistors are made by Kiwame and rated at 2W, all other circuit resistors are 0.5W Takman metal film. Main smoothing caps are Cornell Dubilier SLPX Electrolytics. Output caps are Russian PIO. Wiring is with fine PTFE sheathed copper wire. Valves are ECC802S.

In operation anode voltage is 215v and heater voltage is 5.9v
I am driving into an old Quad 405 amplifier. Source is a 'Pure' Bluetooth received.

The hiss is audible after the 'warm up' of the valves. It rises to a fairly high level over the warmup process. Equally on both channels. Sound quality seems unaffected - it is still outstanding.

I have another similar build in another system which is almost totally silent so I know this is not the way it should be.

Thanks again.

Author:  gofar99 [ 20 Jun 2020, 09:58 ]
Post subject:  Re: SRPP Preamp hissing

Hi, My thought is it is the tubes. I have tried batches of tubes in the same circuit and the noise level will vary as much as 6 db in good tubes. A bad or weak one can double that. A lot of NOS tubes are particularly noisy. (contrary to what is commonly believed) The most consistent ones I have found are the JJ ECC802S. Not the quietest, but normally quiet enough. I have heard of folks being what I call "swindled" by various sellers (individuals and dealers) that they will sell you the quietest tubes. My experience has been if they say that then it is probably not the case. Unfortunately it is a buyer beware situation. I don't buy any tubes I can't return for full refund and even then only from established sources. My two "favorite" sources are (AKA Antique Electronic Supply) and Solen USA. The latter one is for KT120 power tubes only. What I would try is swap the tubes from the quiet preamp to the other one and see if is better. If not then something else it going on that increases the noise. One thing to try is to do something like a "humdinger" with two 100-200 ohm resistors. One from each side of the heater circuit with the junction between them going to the heater lift voltage source. This sometimes will quiet similar circuits a bit. Keep us posted on how it works out.

Good listening

Author:  RevM [ 20 Jun 2020, 10:21 ]
Post subject:  Re: SRPP Preamp hissing

Thanks Bruce,

I'll try swapping the tubes and see where I get to.
As an observation there seems to be a much stronger tube amp community in the US than the UK. It is particularly difficult to get transformers over here (except for guitar amps). I've shipped from the US before now, but it's very expensive. Also we have no High Street electronics suppliers any more, Maplin (part of Radio Shack I think) went under a few years ago. Mouser now have a UK presence, but shipping is still from the US and therefore costly. Ah well.

Thanks again for your help,


Author:  M. Gregg [ 22 Jun 2020, 11:18 ]
Post subject:  Re: SRPP Preamp hissing

Hello Mark,

In SRPP its normal to ensure the heater voltage for both tubes is set at a point to ensure the cathode to heater leakage is not exceeded.

For information here is a strange link for you reminds me of a transformer manufacturer in the 80's that stopped manufacture called Danbury.
Why these transformers look like Danbury I have no idea.

M .Gregg

Author:  RevM [ 23 Jun 2020, 12:52 ]
Post subject:  Re: SRPP Preamp hissing

Thanks, I have just discovered this company. It is one of their transformers I am using.
The heater potential is raised wrt the HT as per Bruce's circuit.
Swapping valves has not fixed the hiss. I am revisiting the voltage regulator circuits. I have had a hissy LR8 circuit before.

Thanks for all this help. I'll keep you posted.


Author:  M. Gregg [ 23 Jun 2020, 12:58 ]
Post subject:  Re: SRPP Preamp hissing

If all else fails try this,

Instead of lifting the heaters with the B+ via a divider.

Try disconnecting the B+ lift from the heater circuit and put a high voltage 0.1 polypropylene from the heater centre tap to ground.

The Cap will charge up from the heater leakage and set itself at an average of the top and bottom tube which may be closer to the leakage voltage of the two tubes. So the heaters are totally isolated floating on the charge potential of the cap and see if it makes any difference. (its an interesting test)

M. Gregg

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