2A3 pre-amp

[pre65]

you will have done that with your resistor network that lifts the DC potential. I presume you bypassed the resistor which goes to ground with a cap?

Yes, a 100uf.

[Paul Barker]

Can't fault it.

Must be you mate

[Dave the bass]

Its the jumper.

DTB

[pre65]

Its the jumper.

DTB

I only wear it on special occasions, like next Thursday.

[Dave the bass]

Do you have any sick bags?

DTB

[Mike H]

To be fair, he did give fair warning in advance

Could've waited to spring it as a surprise....

[pre65]

The modified 2A3 pre, now with the single 6080 or 6AS7ga is a lovely sounding pre. Just incase anyone thinks I'm dissing it by preferring the sound of Andrews 26 pre.

I shall be keeping it intact, and using it until I find something more magical.

My thanks to Nick for sharing the details.

[IDM]

Just to prove how slow I am building anything, today I have finally managed to lash together this linestage.

My version is being tested with Valve Art 2A3's. and Russina 6P14 valves.

The heating of the 2A3's is done using Rod Coleman DC heater modules. The only fast part of this project was the speed that Rod replied to my emails and shipped me the kits (truly first class).

The 6P14's are AC heated and lifted to 200V.

The PSU is: 350-0-350/ GZ34/2uf/20H choke/56uf/220R/220uf. When I first tested I didnt have the last stage of RC and it hummed badly.

However, with the extra smoothing its silent. I get almost no hum or hiss. In fact I can only detect any noise by putting my ear by the speaker cone. I am not sure if I don't get the noise problems due to the gain of my power amplifiers (300b's driven by pentode C3M).

I cannot make any real judgement as to the sonic performance yet as it is lashed up with extremely nasty (freeby interconnects). The only thing I notice is that the 2A3's are microphonic if I clap my hands you can hear vibrations through the speakers. I think the microphonic problem was observed by Brian Cherry during the development of the Tram2.

Will post more observations later.

Cheers
Ian

[IDM]

I have tried many in the past from 12B4A, Aikido and cloned Audio Note M77 with highly regulated powers supplies. I would have to say that this linestage is right up there with the best its transparency and dynamics are spectacular. It seems to really open the soundstage and move everything forward into the room. Vocals are sublime.

I am doing everything I can to acoustically dampen the chassis to minimise microphonics through the 2A3's. Their valve holders are mounted using rubber washers between the valve holders and the chassis and between the valve holders and the mounting bolts. I am also trying a small spot of blue-tack under the valve base between it and the valve holder. This seems to have reduced the initial problems by a huge amount. However, I think there must be even better ideas. I would really value any advice on acoustic damping that other might have.

With the circuit, I measure the current draw as 30mA per channel. I will therefore try 10M45's set to 30mA in place of the 200R resistor between the cathode and G1 of the El84.

I would also like to try regulating the PSU, however my Hammond transformer is only good for 110mA so I don't think I can shunt regulation, are there any good ideas I should try?
Cheers
Ian

[IDM]

I tried removing the cathode bypass caps. This resulted in an annoying background buzz (not mains hum). Why would that be? Is there something funny about DHT's that cause this? Removing caps from other IDHT linestages has always worked ok for me.

Cheers
Ian

[IslandPink]

I would also like to try regulating the PSU, however my Hammond transformer is only good for 110mA so I don't think I can shunt regulation, are there any good ideas I should try?

You might not need to shunt very much - eg. 20ma for both channels, because it's only a few volts signal swing in the 2A3's . You could make an estimate of the max current swing using the 2A3 curves, signal Pk-Pk and estimate of the loading resistance from the 6P14.

[Nick]

I tried removing the cathode bypass caps. This resulted in an annoying background buzz (not mains hum). Why would that be? Is there something funny about DHT's that cause this? Removing caps from other IDHT linestages has always worked ok for me.

Cheers
Ian

Its from the filament transformer. The DC reg doesnt see it as its common mode on the filament supply.. The bypass cap shunts it to 0v, without the cap, its seen by the triode and amplified.

[IDM]

Now that I have tried to tidy up the rats nest that was my 2A3 pre it now doesn't work properly!!

It does play music but with a very pronounced hum. I have tried all the tricks I know for earthing to no avail (it shouldn't have been necessary as it worked before). I have even tried completely rebuilding, still hums.

However the hum disappears when I turn the volume right down (the wiper of the pot putting the grid to ground), As soon as I advance the pot even slightly I get alot of crackle (like there was no grid leak resistor) and the thing hums!!

I tried removing the CD player in case it was the source that was the problem, but it hums just the same.

With the volume pot turned to zero it is more or less silent, from which I assume its not the heaters or the HT causing the problem.

The voltages and currents look about right, 400V HT each channel pulling about 30mA and about 200V on the anode of the 2A3. The EL84 heaters are AC and lifted to 200V.

I built the circuit exactly as Nick described with a 100k from grid to ground. The pot is 100k with the wiper to the grid of the 2A3.

Any ideas have I done something stupid??

Cheers
Ian

[Nick]

Try a 1k grid stopper.

[IDM]

Thanks Nick,

I had wondered about a grid stopper. I will give it a go first thing tomorrow.

Cheers
Ian