Error in power amplifier schematic

[routhun]

Hello All,

Here is the schematic of the power amp section of Kyocera R-861 receiver. It has MOSFETs in the driver stage. I have very little knowledge in the audio amplifier design so please excuse any of my mistakes. I believe there is an error in the schematic. The value of R520L is marked as 180ohms, but I believe it should be 2.2K to have the symmetrical open loop gain for positive and negative swings of the signal. Am I correct? If so will this kind of asymmetry have any impact on frequency response and the distortion of the amp? Should I change that value to 2.2K. Any advice is appreciated.

Thanks,
Routhun

[routhun]

part2 of the schematic

[ilimzn]

No, Q508L is the wrongly marked one, it should be 180 ohms, instead of 2.2k.

[jan.didden]

Originally posted by ilimzn
No, Q508L is the wrongly marked one, it should be 180 ohms, instead of 2.2k.

Q508L is a transistor.

I think if you work out the collector currents, assuming Q505 and Q506 have the same currents, you probably will find that the Vb-to supply of the drivers is different and that the different resistor still gives the same current. But that's from just a quick look.

Jan Didden

[routhun]

Thanks ilimzn!

Hi Janneman,

I can understand Ilimzn is referring to R519L instead of Q508L.

So my question is should I replace R519L with 180ohms? Will it make any difference in sound?

Routhun

[zeonrider]

Please put on this thread real types of Transistors !
Regards zeoN_Rider

[routhun]

Hi Zeon,

I'll upload the actual transistor types tonight (CA, USA time).

Thanks,
Routhun

[Piercarlo]

Originally posted by routhun
Thanks ilimzn!

Hi Janneman,

I can understand Ilimzn is referring to R519L instead of Q508L.

So my question is should I replace R519L with 180ohms? Will it make any difference in sound?

Routhun

I don't know if make a difference in sound but surely it make difference in good working of circuit (which actually, if the R519 value is real and not only - as i think - a print mistake on schematic, is severely unbalanced).

Ciao and merry xmas to all! :-)
Piercarlo

[GK]

Originally posted by routhun
Thanks ilimzn!

Hi Janneman,

I can understand Ilimzn is referring to R519L instead of Q508L.

So my question is should I replace R519L with 180ohms? Will it make any difference in sound?

Routhun

G'day.
Yes, R519L, the emitter resistor of Q508L, should be changed from 2k2 to 180R to balance the currents. However, the designers might have done this as an after thought for a reason. Note that one of the frequency compensation capacitors (C505L) connects to the collector of Q508L. I'm guessing that perhaps the amplifier wasn't stable with the VAS currents balanced, so the gain of amplifier stage formed my Q508L was reduced by increasing the emitter resistor.

Cheers,
Glen

[Piercarlo]

Originally posted by G.Kleinschmidt

Note that one of the frequency compensation capacitors (C505L) connects to the collector of Q508L. I'm guessing that perhaps the amplifier wasn't stable with the VAS currents balanced, so the gain of amplifier stage formed my Q508L was reduced by increasing the emitter resistor.

Cheers,
Glen

Compensation capacitor C 505 compensate open loop gain for *all* VAS circuitry, and not only for Q508. You must see the output network of VAS (apart from VBE multiplier and following output stage) as a parallel of C505, R521 and R522. This pole is rawly settled for a minimun frequency of 21 KHz (in the reality more high due to other "hidden" conductance connected in parallel to ouput: collector output conductance, loading impedance from output stage and so on - this compensation is also aided by a second pole on the differential input stage output which "kill" it's voltage gain above 240 kHz - rawly).
A raw estimation for the real roll-off of this pole may be just a doubled frequency, which let to intend two thing: this amplifier is intended for achieving very high slew rates, extended power bandwidth with light and splitted compensation and relying heavily for it's stability on the "hard" separation operated from the MOS driver between the VAS load and any kind of output load reflected on VAS itself. Is a very sofisticated design and, for somewhat, enough critical to discouraging "lighthearty" intervention. Open loop gain (about 90 dB) is high but prudently limited by local emitter degenerations.

Hi!
Piercarlo