#26 pre amp

[euro21]

My measuring.

Two box (chassis) design.

Power supply box: HT transformer, AZ1 mesh, CLCLC raw supply and two SSHV2, LT transformer, dual LT raw supply -diode bridge, C-CMC-C-L-C), 19 pin military connector.

Preamp box: (dual mono in one box) 19 pin military connector, F.A.D.A #26 tube, DN2540 cascode CCS, heater bias -10R 50W- Rod's regulator, 350nF (220nF FT3 teflon+130nF SSG-3 silver mica) coupler, S&B TX-102 TVC.

Between power supply and preamp box interconnect:

- Right LT supply non twisted 14awg
- Right SSHV2 F+ F0 twisted, S+ S0 twisted 19awg
- Left LT supply non twisted
- Left SSHV2 F+ F0 twisted, S+ S0 twisted

Right channel: B+ 400uV noise, bias resistor: 300uV -average- noise
Left channel: B+ 200uV noise, bias resistor: 500uV -average-noise

No shielding!

Spectrum at 6V RMS output.

[mogliaa]

Hey Euro, this looks really good. Very quiet preamp indeed!
I've been playing today rearranging the HT feed at the preamp but still need to crank up the raw HT (currently at 200V) a bit higher as don't have sufficient headroom between the salas V1 and the CCS feeding the VR (150V).
Despite having only 180uV at the filaments, my output noise level is still high at -70dB

I will tweak the raw HT and salas regulator to reduce input ripple a bit further.
Ale

[euro21]

Originally Posted by mogliaa

Very quiet preamp indeed!

Very quiet tubes.....though not all...

[andyjevans]

I had a bit of a shock today when I switched two different filament supplies for the output stage of my PSE 4P1L amp, which uses filament bias. Not quite the same as a 26 preamp, but similar issues. I started with two bench supplies to get it going and check voltages - standard Thurlby Thandar 30v, 2mA. With these the frequency response was quite even, with good vocals and midrange and rather weak bass. Then I switched to a supply with a huge 300VA toroid and choke input, 280mH, into 10,000 reservoir caps. That was quite a different sound - a lot of bass energy, very punchy but not as delicate in the midband.

I hadn't expected such an audible difference. This is going to make me take the filament supply more seriously!!

Still not sure whether to screen the interconnect from filament supply to the main signal chassis.

Andy

[Magz]

I made an umbilical using Belden 16 gauge stranded silver plated copper in teflon for the + leads and Cardas 15.5ga copper litz for the returns. All twisted pairs. Filaments, B+ and control power (9V) all in same cable, each pair shielded with copper braid, the braids brought together at each end to one connector pin, and that pin is grounded to the chassis at both ends.

Dead quiet (<-100dB background noise, 200uV or so, although Í'm at the limit of my sound card).

Connectors are 8-pole Neutrik Speakons.

[Magz]

Hey, I did some more fiddling today.

I was worried that in my zeal to get the shortest possible signal path I may have run the inputs a bit too close to the outputs, so I built shields to go around the output autoformers, out of TI-Shield, which is permalloy with copper plating on each side, so it handles both magnetic and electrostatic fields. The shields are bolted to the chassis bottom, which is chromated for good conductivity.

After I buttoned it all up I gave it a listen - WOW. The background was a little blacker than before, and instrument separation and detail was noticeable improved! I guess I was getting a little cross contamination before.

Attached is a picture of the shield implementation Well worth the effort!

[Magz]

Originally Posted by bravi

Hi Magz,

Any advantage with using Music rails on the filament supply before Rod's regulator? Any difference in sound without it?

Cheers!

Ravi

While I was in there I did some measurements for you. Voltage drop across the Music Rail is 0.5V, as specified. Noise going into the MR was 36mV RMS, coming out was 10mV RMS. So there is a decent reduction, but not huge.

I put them in because I wanted to get the noise down to the low mV level before the filament wires went into the umbilical next to the B+, and to drop a little voltage before the regulator boards, and I think 10mV RMS is acceptable for that. Rod's boards then drop it much further - dead quiet!

[bravi]

Thanks for the measurements.

Cheers!

Ravi

[Vinylsavor]

Hi Andy

Originally Posted by andyjevans

Still not sure whether to screen the interconnect from filament supply to the main signal chassis.

There are many much more important points than the umbilical. For example a screen (or better two) between primaries and secondaries of your power transformers. I for example avoid toroids.

Of course it will not hurt to use a screened umbilical (Mine have screens). Connect th screen at the PSU end to PSU chassis and saftey earth.

Best regards

Thomas

[Rod Coleman]

Yes, the effect of screening is not very large. But it will be variable, depending on the sources of EM noise nearby.

My personal choice would be to fit the screened cable, simply because it is so easy to do, and when you are trying to achieve the best possible sound, even the smallest things can have a pleasant influence.