2A3 pre-amp

IDM · Post by **IDM** » Thu Feb 21, 2013 10:25 am

Now sorted. The problem seems to be a combination of 2 things:

1) the lack of grid stopper was giving the issue of strange noises from the pot and some of the hum.

2) the output has Obbligato copper sleeved caps, and it the stage hummed when the body of the caps was touching the earthed chassis. (not sure what that would happen) as I presume the body of the cap is insulated from the plates.

I also noticed that one channel had a very quiet whine (like feedback) which got worse when music was playing. I found that the heater voltage was down at 2.24V. The problem disappeared when I adjusted the Coleman reg upto 2.4V. I was surprised that such a difference has such a dramatic affect.

I seem to recall reading that some people think slight under heating of DHT valves improves the sound. does anybody have any opinion as to whether this is true and if so what the sweet spot is?

Cheers
Ian

IDM · Post by **IDM** » Thu Feb 21, 2013 10:47 am

When I first put the grid stoppers in I was in a rush and stupidly left the gridleak resistor on grid pin and simply put the signal input through the grid stopper. This obviously puts the grid leak on the wrong side of the grid stopper.

When I plugged it in it works fine, but it sucks all the life out of the music. It sounded dreadful. It was only when I was pulling the resistors out that I realised what I had done, swapped things round and the music returned. Another lesson learned the hard way!

Cheers
Ian

Post by **Nick** » Thu Feb 21, 2013 11:22 am

the lack of grid stopper was giving the issue of strange noises from the pot and some of the hum.

Typical symptoms of HF instability.

I also noticed that one channel had a very quiet whine (like feedback)

May have been the supply oscillating, the tent labs will do that if they are on the edge of regulating.

IDM · Post by **IDM** » Thu Feb 21, 2013 4:02 pm

This line stage is driving me mad.

It was fine all morning. So I put the power socket on the back, plugged it back in now the feedback is back just for one channel. The feedback note is similar to the old test card note (for those that remember them!!).

From power on its ok, but after a minute it howls. I thought it might be feedback through microphonics but I have tried holding the 2A3 glass envelope and it doesn't damp it.

I have 1.5K kiwame grid stoppers on both 2A3 grids. The wiring between the input grids and the output cap are tight in that they are close together but I have never had anykind of electronic interaction with anything else I have built.

I had some other 2A3's so I tried swapping the valve in case that was the problem, but no difference with different valve. I will try swapping the EL84's see if that tells me anything.

Any ideas where else should I look for the culprit?

I am close to cracking with this project, its just that when it works it sounds so good!

Cheers
Ian

IDM · Post by **IDM** » Thu Feb 21, 2013 4:11 pm

Just swapped the EL84's over and the problem stays on the same channel

Ian

IDM · Post by **IDM** » Thu Feb 21, 2013 5:58 pm

The HT was coming in at the back passed the offending valve to the front where I have a dropper resistor and last PSU cap prior to going into the EL84. If I don't run the HT in from the back but use a lash up from the front the feedback is less pronounced, though it does eventually start. Once started it generally dies though once or twice it has gone into uncontrolled positive feedback and I have to switch everything off as it becomes deafening and I worry about my speakers.

I am now totally stuck. When it works it sounds fantastic, one channel is totally silent and behaves perfectly. The other channel has all the feedback issues.

I have attached a photo in case someone can spot a layout issue. The channel that has the problems is the one at the top of the photo.

Mike H · Post by **Mike H** » Fri Feb 22, 2013 3:50 pm

IDM wrote:I had some other 2A3's so I tried swapping the valve in case that was the problem, but no difference with different valve.

Seems like the circuit is oscillating then, i.e .it's not some issue with a specific valve.

I have 1.5K kiwame grid stoppers on both 2A3 grids. The wiring between the input grids and the output cap are tight in that they are close together but I have never had anykind of electronic interaction with anything else I have built.

By doing that you might have unwittingly made a Colpitts oscillator. Which uses capacitive coupled positive feedback between cathode and grid. If you had say a pocket FM radio you might be able to test if it's transmitting. The connecting wires become the 'coils'.

Try moving them far apart instead see it that helps.

This is an ever present danger with output type valves because the currents can be high, and the impedances low.

HTH

IDM · Post by **IDM** » Sat Feb 23, 2013 2:57 pm

Finally seem to have got it tamed, no more nasty noises infact its very quiet. The only issue being the microphonics of the 2A3's. I would like to try some different brands.

Cheers
Ian

IDM · Post by **IDM** » Fri Mar 01, 2013 5:53 pm

I Have finally got thus circuit reliable other than the mcrophonics. Having looked round the web I see that some people put an attenuator on the output of the 2A3 (after the coupling cap). This has the double benefit of reducing the gain, thus reducing the microphonics and also driving the 2A3 harder (which apparently should make it sound better).

The attenuator circuit I looked at (from the Tram2) consisted of a 2.4K resistor from the output cap to the centre pin of the output pin of the phono plug and a 1.4K resistor across the output phono to ground. This simply makes a potential divider on the output to drop the gain.

It all seemed very easy until I tried it. As soon as I put a 2.4k resistor from the output cap to the phono socket the stage hums badly (again!!). Is there a simple reason why this hould happen or is there something still quirky about how I have built the linestage?

I would really appreciate some thoughts on this as I think I am very closer to getting this pre-amp sorted.

Cheers
Ian

IDM · Post by **IDM** » Wed Mar 06, 2013 1:34 pm

It struck me that why use an attenuator on the output of the pre-amp when I could drop the gain of my power amps by removing the cathode by-pass caps from the C3M driver.

So tried it yesterday and the microphonics have now almost completely gone. Everything is now almost completely silent. It sounds fantastic and two less electrolytics! I think this is finally totally sorted.

Cheers
Ian

Post by **Nick** » Wed Mar 06, 2013 9:11 pm

Good solution, better to reduce gain than to attenuate down and then amplify up again. I did the same and removed a stage from my 211 amp.

Mike H · Post by **Mike H** » Thu Mar 07, 2013 1:23 pm

Ah yes I'd forgotten you said that earlier. There ya go then.

Clive · Post by **Clive** » Thu Mar 07, 2013 2:26 pm

Once the gain structure of the system is setup I can't help thinking that with a DHT preamp the valve should see the full signal and the volume control (AVC?) Should be on the output of the preamp. Just my 2p.

Clive · Post by **Clive** » Thu Mar 07, 2013 2:27 pm

Once the gain structure of the system is setup I can't help thinking that with a DHT preamp the valve should see the full signal and the volume control (AVC?) Should be on the output of the preamp. Just my 2p.

Post by **Nick** » Thu Mar 07, 2013 8:58 pm

Clive wrote:Once the gain structure of the system is setup I can't help thinking that with a DHT preamp the valve should see the full signal and the volume control (AVC?) Should be on the output of the preamp. Just my 2p.

Yep, but its the effect that will have on the output impedance that puts me off the idea.

audio-talk

2A3 pre-amp

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