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madtecchy 15th September 2012 06:18 PM

Front end help
 
1 Attachment(s)
hello.
Would somone please tell me the name of the front end topology of this amplifier?

Best Regards Mark

jkuetemann 15th September 2012 07:07 PM

I suppose one could call it 'Symmetrical Differential'. It's just two normal resistively loaded LTPs of opposing polarity.

madtecchy 15th September 2012 07:23 PM

Thankyou Jason.
could you tell me the main pros and cons of such a configration . compared to a sigle LTP??

Ian Finch 15th September 2012 09:51 PM

I think that's an attempt at a push-pull VAS (Q7,8) as well. You will see this in many high-end commercial designs where symmetry is the approach to a certain sound quality (H2 prominent) and, of course, the aesthetics too. e.g. Rotel have favoured it consistently.

jkuetemann 16th September 2012 04:41 AM

I think as Ian mentioned the double LTP is generally used for driving a 'push-pull' VAS. The symmetry, if well executed, seems to offer what some might say a palatable character to the sound. A certain amount of harmonic cancellation can be achieved this way. Cons as I see them really amount to added complexity. One could achieve really nice results without basically doing everything in the front end x2.

madtecchy 16th September 2012 05:22 AM

Thanks Ian .
This amplifier is from a budget range. yes the layout is quite symetrical too. if the componets where soldered in with some care it wouldlook pretty good. Asafter a few mods it is sounding much better . i have disconected the Preamp / control PCB as it was noisey and strangled the sound . I have also removed the dc blocking cap from the input of the power amp as my dac has no or ver yvery little dc on its output. That made a big diffrence most noticable was a much welcome reduction in sibulance type distortion .

madtecchy 16th September 2012 05:42 AM

Hi Jason
Added complexity normally means added cost or so i thought. Im guessing it is an attempt to steal away business from the more prominent manufacturers. inside does look quite tidy compared to others i have seen in this price range ... A couple of questions . This amp runs cool very cool and the heatsink is very small too . I suspect it is running a quite low bias . maybe it could be increased . Also i dont see any compensation capacitors well that is if i ignore the two 100nF film caps . if the 100nF caps are for compensation are they not quite high in value as normslly i see the caps with only say 30 pF in these. C4 and C5 are the capacitors in question.

madtecchy 16th September 2012 06:20 AM

in the above post i mention 100nf it should be 100pF

Dave Zan 16th September 2012 01:57 PM

Benefits of complementary LTP.
 
1. Potential for reduction in input noise by up to 3dB.
2. Can be easier to control DC offset with cancellation of input currents - dependant on effort to match complementary Hfes.
3. Potential to reduce distortion.
4. Easy way to drive push-pull VAS.
5. It looks nice - to many people at least

There are drawbacks too. See Bob Cordell's book for a more detailed analysis. He is not a real fan of this layout.

Best wishes
David

jkuetemann 16th September 2012 04:15 PM

Quote:

Originally Posted by madtecchy (Post 3166748)
Hi Jason
Added complexity normally means added cost or so i thought. Im guessing it is an attempt to steal away business from the more prominent manufacturers. inside does look quite tidy compared to others i have seen in this price range ... A couple of questions . This amp runs cool very cool and the heatsink is very small too . I suspect it is running a quite low bias . maybe it could be increased . Also i dont see any compensation capacitors well that is if i ignore the two 100nF film caps . if the 100nF caps are for compensation are they not quite high in value as normslly i see the caps with only say 30 pF in these. C4 and C5 are the capacitors in question.

Small signal stage costs are generally small compared to power supply, heat sink and chassis. It adds to the aesthetic and make some people feel all warm and fuzzy inside.

Q8 looks like the emitter and collector are reversed. C4 and C5 would be plain old miller compensation caps and 100pF isn't necessarily out of line, depending on LTP and VAS currents. The emitter resistor values in the VAS degeneration also don't appear correct, and are not equal.

Also absent is any emitter degeneration of the LTP and an input filter. The feedback network values look suspect (40dB gain) and there is no input resistor to allow for bias currents.

As for bias, it may well be somewhat low. If the heat sink is particularly small I wouldn't try to bias it too heavily. Typical values for 'optimal bias' puts about 26mV across each resistor R26 and R23. Trying to make the class A region too big will result in a hot amplifier and gm doubling which some say is every bit as bad as under-biasing.

Is it safe to assume you have drawn this by tracing the physical circuit? That might explain some of the discrepancies...


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