JamJar: an HPA-1-inspired power amp

[JeffYoung]

Based on the success of my HPA-1 clone, I've been playing with a sim of an HPA-1-esque topology with 3 pairs of laterals for the output stage.

It's designed to go into a case built around @mbrennwa's Aleph-style heatsinks. I'm thinking those should be good for at least 0.17ºC/W (with 2 for each channel).

Any thoughts appreciated.

Cheers,
Jeff.

 

[2 picoDumbs]

Increase C4 and C9 dramatically and reduce R22 and R23, eg 15,000uF or greater and 1 Ohm resistors

I personally prefer the Sony Amp front end, and keep your output stage the same

 

[jam]

Jeff,

That is not quite the best way to use that topology for a power amplifier, but it should work.

Hint. Early effect................

Jam

JamJar.....has a ring to it

 

[JeffYoung]

Hi Jam,

Yeah, I thought JamJar went nicely with my "Beefcake" project based on a beefed up BA-2.

Doing my homework on Early Effect. I presume we're talking about the BJTs in the current mirror? (There's evidently something called Channel Length Modulation in MOSFETs, which has similar effects and is therefore sometimes referred to as the Early Effect, so I just want to make sure I'm not chasing the rabbit down the wrong hole.)

Cheers,
Jeff.

 

[Mark Johnson]

If you changed the 3-Transistor Wilson Current Mirror (Q5, Q6, Q7) into a full 4-Transistor Wilson Current Mirror, and if you used the same TO-126 part number for all four transistors, then you might get better cancellation of base current errors.

It might be worth considering the KSA1220A instead of the KSA1381; the A1220 has less quasi-saturation at very low VCE, which is where Wilson Mirrors operate.

_

 

[jam]

Jeff,

The key being the correct topology for the job.

Pico mentioned Sony had a good topology on some units one of the best being the schematic attached. Sounds great and in Sony's case the output stage had a lot to be desired and I would leave out the mirror on the differential.

Jam

 

[jam]

Current Mirrors

Some notes on mirrors.....

A four transistor mentioned by Mark measures slightly better but does not seem to sound any different (except a brighter top end) so I did not use it in the HPA-1.

Jam

 

[JeffYoung]

I think I'm beginning to understand what Jam meant about the topology. I tried increasing the current mirror resistance to drop the Vce (and therefore the Early effect). However, once you drop the voltage you no longer have enough to swing the output devices near the rails.

I did manage to drop the THD by about 80% (!) by increasing the current mirror resistance to 330R : 100R and bumping the rail voltage by 2V. (Hey, it's just a bit more heat to burn off, right?) I don't think this had anything to do with the Early effect because the two changes play against each other and leave Vce only very slightly lower.

I was much less successful homogenising the current mirrors. The TO-126s just don't have enough gain. The KSA1220s are better than 1381s, but to equal the closed-loop gain of the hybrid TO-92/TO-126 mirrors I had to reduce the feedback, frittering away all the THD gains of the homogenous current mirrors.

I think the next step is to try out the MOSFET second stage from Nelson's Sony VFET part 2 article. I'm sure the Sony schematic Jam posted has merit, but I struggle to even make heads or tails of it.

Cheers,
Jeff.

 

[2 picoDumbs]

There are a lot of good things to play around with, with the Sony front end..

 

[jam]

Jeff,

Regarding the Sony schematic,
1) Strip it down to the bare minimum
2) Redraw
3) Add elements that help and discard the rest

......big hint as mentioned leave out the mirror oh top of the differential, replace R408 with a trimpot to correctly bias Q405.
or just replace Q408 with a bipolar.

This should be an interesting exercise.

Jam

 

[Itsmee]

I like the BJT Cascode Current Mirror, page 21

 

[EUVL]

See also :
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun.html#post5053658 (post #11)
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun-2.html#post5230740 (post #89)
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun-3.html#post5679200 (post #116)


Patrick

 

[Itsmee]

Don't know if it would help, but this is where I am with mine - almost, double JFET on the next one.

 

[EUVL]

Your cascode voltage setter can be much simpler.
Your Wilson mirror with BJT is not connected correctly.
There is not a lot of benefit using dual JFETs.
And unless it is for headphone, the output devices are not powerful enough for loudspeakers.


Cheers,
Patrick

 

[Itsmee]

Your cascode voltage setter can be much simpler.

Yes, tried most variations, just wanted to see if I could here a difference to the TL431

Your Wilson mirror with BJT is not connected correctly.

It's not a Wilson, see page 21 of the PDF in post 11

There is not a lot of benefit using dual JFETs.

Thank you, I did wonder that, I don't know if I don't try it, I'm not spice compatible yet, working on it, and I don't have a formal education in analogue design, I'm just a knuckle scraper.

And unless it is for headphone, the output devices are not powerful enough for loudspeakers.

That's what the Little Pucker is for.

I have been playing with variations for years, I found the Knut Harald Nygaard version about 5 years after my first prototype, his predates mine by about 6 years!

Seems I reinventing the wheel (again), I usually find the original years after I've done the hard work!

 

[EUVL]

The current on both legs of your mirror is not balanced due to base current.


Patrick

 

[Itsmee]

That's page 22, page 21 states Because VCE1= VCE2, this mirror will have very good matching if βF is very large. The BC327-40 has a minimum hfe of 250, is that not good enough?

Forgot to add, have a look at the summery on page 32.

 

[Markw4]

See also :
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun.html#post5053658 (post #11)
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun-2.html#post5230740 (post #89)
https://www.diyaudio.com/forums/headphone-systems/306350-uthaim-fun-3.html#post5679200 (post #116)

Turns out that one can link to individual posts. Just right click on the post number at the upper right corner of a post and select 'copy link,' or left click on the post number and it will open a new window with that post at the top of the page. Then you can copy the link address from that new window.

Perhaps that is already what you were trying to do, but the links don't go directly to the posts in parenthesis for me.

 

[EUVL]

> but the links don't go directly to the posts in parenthesis for me.

Because I use 50 posts per page, and you probably not.


Patrick

 

[Markw4]

Ah, yes. An interesting feature of the forum software. But then, why offer individual post links at all without a mention they probably won't work for default page format users?