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Realistic 210 reborn (AKA Acrosound UL120)

That's simply fantastic.

I wonder about the HF rolloff- I think I remember Dave Gillespie, or someone at least, mentioning that there were some slight tweaks that could be done to the compensation networks of the Acro 120, to make it basically flat up to 20KHz, but without degrading the stability significantly? Here's hoping anyone who remembers that better, will chime in here...

Regards,
Gordon.
 
GordonW, I am unaware of any mods to this amp. What compensation networks are you referring to?

Rustnrot, I use Sprint-Layout 6. It allows you to insert a picture to the background of your board.
So with a scale 1:1 picture, you can copy easily an existing layout.
Then you optimize by hand on 2 layers, then generate the gerber and drill files with that software.
 
If this shares the same circuit as the UL120, there looks to only be one phase/amplitude compensation circuit- a 47K in series with a 100pf cap, between the first and second gain stages. That acts to reduce the HF gain of the amp a bit, at a shelf center frequency of about 34KHz.

That said- I can't find any indication of anyone who has done any tweaking to one of these- so I may well be remembering a different Acro model.

Regards,
Gordon.
 
thing I've found with those shelf networks, the calculated value doesn't always represent exactly what they do. The additional load on the plate of a high resistance tube like a 12ax7 makes them start operating lower than their values would indicate.
 
I've seen several European variations of Realistic branded equipment over the years, but nothing like that amplifier. The examples I have seen were versions of equipment sold in the US but with more features, higher output in the amplifier sections, etc.
 
GordonW: Thank you. Yes I agree. Reducing C or increasing R may extend the response better in higher frequencies.
I quickly ran a sim on it and it shows it when the 100pf cap is stepped from 10 to 200pf. Now that needs to be validated on the real world.
UL120-simple.jpg

Gadget: Thanks, good point as usual. Thank you.

Kappclark: It sounds superb. I prefer the EH 6CA7EH on it. The drivers are Sylvania. You guys like the Allen branding on the 12AU7 :rolleyes:.
The UL120, like the ULII, are fantastic amplifiers. They are extremely detailed, well integrated top to bottom, fast but non fatiguing.
Once you listen to one, it's dangerous as extremely addictive :biggrin:
 
Thanks Jack.

Scott, when I worked on the ULII, I didn't do that exact measurement. I did the freq response a full power, attached.
I'll have to do the exact test, as it's easy enough with the Pico to reproduce the exact same test.
I'll do it soon.

ULII-EL34-Tests.jpg
 
I did some measurements tonight when changing the 100pf caps of the compensation network, and reducing its value.
Yes the frequency response improves.
These are on the ULII as it's easier to work with a much lighter amp :)

C=100pf:
ULII.jpg

C=68pF:
ULII-68pf.png

C=56pF
ULII-56pf.png

C=36pF
ULII-36pf.png

The square response with C=36pF at 1Khz and 10Khz are still very good.
ULII-EL34-1K-36pf.jpg

ULII-EL34-10K-36pf.jpg

So what you guys think about this? Should I keep the 36pf and do a listening session?
 
That does look quite good. Worth a listen certainly. Maybe split the difference at 47 if its a smidge too bright.
 
gadget73 said:
That does look quite good. Worth a listen certainly. Maybe split the difference at 47 if its a smidge too bright.
Click to expand...

I agree. Brice, that looks a lot better. Seems like the original circuit was a bit over-damped. Since you're using the Picoscope, what do the phase margins look like?
 
I would run no-load stability tests (capacitive only load) with the smaller caps, to make sure it isn't ringing too much. Dave G. has written quite a bit elsewhere, on what values to do, but IIRC- as long as you can connect a 0.1uf cap across the output without it ringing badly (no blatant oscillation cycles on top of the square wave), it should be fine.

I'd bet you will be able to go down to 47pf, but I'd want to make sure that it's still stable enough.

Regards,
Gordon.
 
Thanks guys,
I did a HF stability test. Square signal input, caps load only from 0.05 to 1uF. The amp is totally stable.

Load 0.1uF:
36pf-0.1uF-10k.jpg

Load=0.5uF
36pf-0.5uF-10k.jpg

Load 1uF
36pf-1uF-10k.jpg


On my bench it sounds very good, but it's hard to say.
I guess now I have to set my preamp in mono, then try this amplifier and an unmodified one and alternate the channels with the balance to try to hear a difference.
 
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