Pass HPA-1, what do we know?

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A picture is worth a thousand words...

An externally hosted image should be here but it was not working when we last tested it.


Zooming in on the left channel, I see 12 active devices in a very symmetric layout. It looks like 10 actives prior to the two big output MOSFETs with large 3w source resistors. I see one DC offset trimpot, 1 bias trimpot. There is a TO92 programmable voltage source (TLM431?) providing bias current adjustment, it's not in the audio path. The opamp is in between the two channels and I don't think it is in the audio path either.

583104d1480545185-pass-hpa-1-what-do-we-know-pass-ha-crop.png


I suspect it might be a push-pull output stage with something resembling one of Juma's creations.
 

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> Do we know anything about the design?

More than you need to reverse engineer it, even down to component level.
Read the 6moon review in detail, not only the pictures.

> Push-Pull or single-ended?

Single-ended (as opposed to balanced) push-pull.

> Based on any currently known topology (Aleph, etc.)?

No.

> 5 pairs = 4 beasts and a cascode?

6moon :
"...... I tried different devices including bipolars and in the final analysis we chose Toshiba Jfets for the input stage,
Fairchild Mosfets for the outputs and bipolars in the current mirrors
so we ended up using all three device types.....
the voltage gain occurs in the Jfet/mirror stage and the Mosfets provide the current drive....."


Patrick
 
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> Do we know anything about the design?

More than you need to reverse engineer it, even down to component level.
Read the 6moon review in detail, not only the pictures.

> Push-Pull or single-ended?

Single-ended (as opposed to balanced) push-pull.

> Based on any currently known topology (Aleph, etc.)?

No.

> 5 pairs = 4 beasts and a cascode?

6moon :
"...... I tried different devices including bipolars and in the final analysis we chose Toshiba Jfets for the input stage,
Fairchild Mosfets for the outputs and bipolars in the current mirrors
so we ended up using all three device types.....
the voltage gain occurs in the Jfet/mirror stage and the Mosfets provide the current drive....."


Patrick

Sound like this one: http://gilmore.chem.northwestern.edu/lc.pdf
 
In is a current feedback design, the IC is used as a servo and Juma is on the right track.

Circuit design aside part of the key is the custom transformer, which surprised me in the design phase on how much transformers affect the final sound. :)

Jam
 
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Hi Zen Mod,

I agree with you completely, but with the HPA-1 heat would have been an issue. I have a pre-amp prototype that I am working on that uses a shunt. I run about 1.5 time more current through the shunt than through the circuit for best sound.

A pic
 

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