Building a valve amplifier

[horias2000]

Oh, if there's a pot involved, the imbalance you're getting is better than usual. Stepped attenuators might be a touch better (at a huge cost and size premium), but only a touch. You have a good result there.

Yes, there's a pot involved
I've added a pre-amp before the power amp stages so that I can exploit the entire power for different input sources. It's a simple schematic shown below.

It goes from about -75dB to 10dB. I did have some issues with the pot as I used some recycled pots from previous projects and some were defective (probabbly too much abuse when desoldering them from other boards). I will replace the pot at some point just to see if there is a difference. Bunt not anytime soon as I do not have the time right now. It sounds like a great activity around Christmas

 

[horias2000]

So, what do you think about it now? I built a kit WAD KT88 amp in 2001. Many prefer 6550 tubes, you may well prefer the mids.

For now I'm still using the JJ tubes I used during the development of the amp. These measure pretty good and sound very nice. I did look for other options form different manufacturers but at the moment I didn't buy any. I was looking at Genalex Gold Lions, Psvane and TAD. I know there are a lot of discussions around brands of tubes but I did not notice any difference. I built another amp like the one presented here, for a friend of mine and I used PSVANE KT-88s and 12AX7s and EH 12AU7s. The amp did measure a bit better in thd and thd+n (2-3dB better) but I could not hear the difference. I did notice though that the JJ tubes use higher quality materials. The glass is thicker and the pins seen stronger. But as a more general question, what brands of tubes do you guys recommend?

 

[gwing]

I don't know exactly, probably that

(Toroidal transformers)

are more prone to some external interference, as I understood. But I am not sure if it is true, just quoted opinion.

I'm late to the party here but this prompted me to check some of my reference material. Transformer design (and manufacture) is incredibly complex but very selectively paraphrasing and condensing bits from Morgan Jone's book regarding audio transformers:

"Traditionally superior cores were made as C cores ... (which) was an expensive process and inaccurate assembly could create an air gap. The more modern approach is to use a toroid ..... . Although sectioning the windings on a C core is relatively easy it is very difficult on a toroid, winding geometry on a toroid is quite poor so it is easy to lose the benefits of a better core by having a poor coil."

i.e. you can have good or bad audio transformers as toroids or not. All transformers with good bandwidth, low distortion and good power handling are going to be *very* expensive though.

 

[Down South]

I'd forgotten that I had posted on this thread. If it's not too late, it's always a good idea to include a soft start module. The pot has a major influence on sound quality, if you have used something like an Alps Blue then it is definitely a negative unless you shunt it. Take a look at World-Audio Designs and on the FAQ section is the schematic for shunting a pot. Using Vishay Z foil totally transforms the sound - in a word clarity. Your also living in Eastern Europe so you should be able to easily get some ridiculousy cheap K73-16 caps for coupling use. Same as per Z foil - clarity. There is an old thread on diyaudio about these caps forget all the other Russian caps, these are the ones to look for. Buy the Z foils from Texas Components, elsewhere they are even more expensive. Don't listen to those who have never used these components.

 

[SIY]

The pot has a major influence on sound quality, if you have used something like an Alps Blue then it is definitely a negative unless you shunt it… Using Vishay Z foil totally transforms the sound - in a word clarity.

Any data to support these remarkable claims?

 

[horias2000]

I'd forgotten that I had posted on this thread. If it's not too late, it's always a good idea to include a soft start module. The pot has a major influence on sound quality, if you have used something like an Alps Blue then it is definitely a negative unless you shunt it. Take a look at World-Audio Designs and on the FAQ section is the schematic for shunting a pot. Using Vishay Z foil totally transforms the sound - in a word clarity. Your also living in Eastern Europe so you should be able to easily get some ridiculousy cheap K73-16 caps for coupling use. Same as per Z foil - clarity. There is an old thread on diyaudio about these caps forget all the other Russian caps, these are the ones to look for. Buy the Z foils from Texas Components, elsewhere they are even more expensive. Don't listen to those who have never used these components.

I can't say I heard or measured any differences between different types of photometers. I've used ALPS and Bourns and other ones but there were no differences. For me it's unclear where the differences would come from. I'm an electronics engineer and I like to be able to measure things. If I can't measure I'm ok with listening to differences but in order for these to be audible these really need to be big. So if the differences are big, then most probably these can be measured. I know that pot values have an effect on frequency response if these are really high (50-100k) and that you should design the circuit so that no DC goes through the POT as this will create noises when you adjust that pot. Potentiometers will also generate noise as resistors do, due to thermal noise, as all resistors do. This noise can be easily measured. It could be that a different material will generate less noise than generic carbon potentiometers as metal film resistors generate less noise than thick film.

 

[horias2000]

(Toroidal transformers)

I'm late to the party here but this prompted me to check some of my reference material. Transformer design (and manufacture) is incredibly complex but very selectively paraphrasing and condensing bits from Morgan Jone's book regarding audio transformers:

"Traditionally superior cores were made as C cores ... (which) was an expensive process and inaccurate assembly could create an air gap. The more modern approach is to use a toroid ..... . Although sectioning the windings on a C core is relatively easy it is very difficult on a toroid, winding geometry on a toroid is quite poor so it is easy to lose the benefits of a better core by having a poor coil."

i.e. you can have good or bad audio transformers as toroids or not. All transformers with good bandwidth, low distortion and good power handling are going to be *very* expensive though.

The transformers I got from TOROIDY are excellent. I've bought the SUPREME version with metal casing on top. Frequency response is really good at both the low and high end and THD is really low for a tube amp (as it can be seen from the measurements). I did a lot of testing and measurements and I did not saturate the core due to DC offset on the output of the KT88s. The amp runs really nice for almost two years now. I also built another one with the same output transformers and this one works very well also. The bottom line is that these transformes are really nice and the price is OK.