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PostPosted: 21 Oct 2013, 19:28 
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I was wowed by a tube Woo Audio HP amp at a recent AV show and vowed to build my own. After looking at a number of schematics, main OTL types, I realized that any low power tube power amp could drive phones OPT or not. I wanted to try the idea. Woo claim their amp is transformer coupled and my 1920s UX-171A power amp is transformer coupled from input to out no resistors or capacitors (see the thread on this forum). Beat that Woo!

I hooked up the AT Air phones and Cowon S9 player with a restive wheel in series with the phones to see what I would get. OK I got hum but I also got great sounding music which remind me of the Woo experience (I don't have the phones they were using which were about twice the price of mine). If I now build another UX-171A amp with a much quieter PS and all tranni coupled like my Intermezzo amp not only will I have a very unique HP amp but re-use the unusual tranni only design that I felt worked so well with Intermezzo.

I started with the series resistor from the OPTs set to 10K but quicly brought it down to about 10ohms for full volume on my Cowon player. Some resistive switching in the new HP version should help set max volume with different players. The new amp could also drive speakers to low level like my current power amp does. I noted the Phiaton MS400s had a heavier bass than the better balanced AT Airs.

Here are some images of the crude test.
Attachment:
171HPAmp.jpg
Attachment:
171HPAmpII.jpg


Something I tried after I submitted this article, to see where the hum is coming from, was to lift the top plate away from the three transformers below to find the hum fell away. I may try putting some magnetic shielding between the bottom and top sections of the amp.


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PostPosted: 22 Oct 2013, 21:56 
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Hi mark, If you can find a source for Mu metal sheets they will really help with shielding. I found some made by 3M that had an adhesive backing. Great shielding, horrible to work with. Best cut with really sharp shears. Regardless the stuff had razor sharp edges and never seems to bend exactly like expected. A real challenge to drill it needs to be sandwiched between other stuff to keep from deforming or shredding.

Good listening
Bruce

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PostPosted: 23 Oct 2013, 07:20 
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Bruce, should this build go ahead I will make the PS both very quiet and remote. Though I added extra caps to my current 171A amp it didn't get any quieter. Noise sits around 1.5mV RMS. I need a new build. Too loud for phones.

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PostPosted: 23 Oct 2013, 09:49 
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Hi Mark, I didn't check on the tube type....but if it is a DHT then you may want to do things to the heater circuit. Elevating it can help and then adjusting the grid voltage to match and so on. Noise is one of the main reasons I tend to stay away from DHTs. Yes they can be made quiet...but it complicates the type of designs I cook up.

Good listening
Bruce

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PostPosted: 23 Oct 2013, 17:21 
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gofar99 wrote:
Hi Mark, I didn't check on the tube type....but if it is a DHT then you may want to do things to the heater circuit. Elevating it can help and then adjusting the grid voltage to match and so on. Noise is one of the main reasons I tend to stay away from DHTs. Yes they can be made quiet...but it complicates the type of designs I cook up.

Good listening
Bruce

I added an addition 100uf to the existing 100uf final PS cap (in Intermezzo) but it doesn't seem to have done much. I'll leave it in it can't hurt. The heaters here are DC from regulators. At 1.5mV of noise you would have to admit that is low and as stated elsewhere only by putting my ear right in the cone of the Fostex can I hear a slight hum. On bassy phones though it is too heavy. On better balanced phones, bearable.

Next PS I will build much quieter.

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