Mirror topology (Rotel RB971)

[audio_tony]

Hello Everyone,

Today, in a moment of extreme boredom, I decided to dissect my Rotel RB971 MKII Power amp, and map out the schematic (which is hopefully attached). What I would like to know is, does anybody know the purpose of C616 / R658 (560p / 220R) - is this for 'miller' compensation? Also, what is the purpose of R654 / C618 / C606(in the differential input stage)? Please ignore the transistor types here - I know they are not the original ROTEL items, but my spice software insists on some kind of parameter (can only be selected from a drop down box, I can't enter a manual value!).

Thanks!

Tony.

P.S. Sorry if this posts twice - the first time I got a warning about the omage being too large - but then no mention of whether I should repost or what???

[GEirin]

Hello all.

What is the purpose of c610 (330)

[GEirin]

Hi.
What are the purpose of c610 (150p); R624 (33k); c612 (330p) and c616 (5pf, phase lead cap)?
Thanks
Guillermo

[vynuhl.addict]

Hi Guillermo,


C610 seems to be a form of frequency compensation for the VAS, not typical lag comp but typical for Rotel, R624 loads the VAS output before the drivers, crude and simple and C612 seems to be another part of compensation, a low pass filter to shunt HF to ground. C616, you are right in it being phase lead compensation.



Best Wishes
Colin

[audio_tony]

I have posted a clearer rendition of my original schematic, as the export I did obscured some of the circuit.
I have also added some additional comments.

What is interesting to note, is that in a spice emulation (yes, I know spice results are not to be taken to seriously!) - is that with C616/R658 and C614/R656 in circuit, there is a gradual rise in HF response beyond about 30khz, resulting in a huge peak at 328khz. Removing these components results in a flat response...
I'm also aware that the component values used for the spice emulation are not those in th eoriginal Rotel circuit.
It was just interesting to note the response, as the amp has a tendency to sound a little sharp at times, so I wondered if this could be a contributing factor.

[audio_tony]

Oops! Attached the frequency response graph instead of the revised schematic!

Here is the revised schematic.

[GEirin]

HI Colin.
Thank for you reply.
Guillermo.

[ppa]

this is the classic rotel schematic, the compensation system do not use miller compensation, there are 3 caps for retard compensation for dominant pole.

[ppa]

the schematic is classic but the compensation system isn't so simple like it is showed.

[ppa]

I forgotten that it's possible make a clone with transistors easy to find in europe, but it's possible make some mods for decrease the very hight NFB.