musical fidelity p170 power amp

whats gan on · 27th December 2010, 12:37 PM

i have modified my p 170 using AD979 op amps this makes a big improvement to the sound but when i unplug the interconnects the amp goes in to motor boating with repeated low frequency popping from me martin logans does anyone know how to stop this also i want to replace the two main audio transformers does anyone know the va rating of these transformers thanks

indianajo · 05:32 PM

You don't say what op amp it had before or provide a link to the schematics. A lot of times when you put in a faster op amp than a 741 or a 4558, oscillation occurs. This happened on my disco mixer at 1 Mhz. It didn't cause my CS800S amp to motorboat, but it sure ran the cooling fan really hard. You might try putting xx pf (picofarads) of capacitance around the feedback resistor on the op amp. This cuts the gain of the circuit at high frequencies. I used disks marked 22 from a R**** S****** grab bag, probably about 22 pf, and probably mismarked, knowing R**** S******. No more oscillation.
The feedback resistor is the one that runs from the point of the op amp (output) to the minus input.
You might also put in a shorting jack on the input of the amp if it is 1/4 phone or mini phone jack. This shorts the input when the source plug is not attached and prevents picking up RF from from the PC or TV in your room, etc.

jaycee · 28th December 2010, 01:00 AM

you also want power supply decoupling capacitors as close to the opamps as possible. 100nF from each rail to ground, and 100uF between rails.

Honestly, in a circuit like this, you're probably not getting much over using NE5532's

whats gan on · 15th January 2011, 05:16 PM

thank you for the advice i will try that i bought a single version of the lm4562 and it sounds to my ears as good as the ad 797 and no problems

27th December 2010, 12:37 PM	#1
whats gan on diyAudio Member Join Date: Oct 2006	musical fidelity p170 power amp i have modified my p 170 using AD979 op amps this makes a big improvement to the sound but when i unplug the interconnects the amp goes in to motor boating with repeated low frequency popping from me martin logans does anyone know how to stop this also i want to replace the two main audio transformers does anyone know the va rating of these transformers thanks

27th December 2010, 05:26 PM	#2
indianajo diyAudio Member Join Date: Jan 2010 Location: Jeffersonville, Indiana USA	You don't say what op amp it had before or provide a link to the schematics. A lot of times when you put in a faster op amp than a 741 or a 4558, oscillation occurs. This happened on my disco mixer at 1 Mhz. It didn't cause my CS800S amp to motorboat, but it sure ran the cooling fan really hard. You might try putting xx pf (picofarads) of capacitance around the feedback resistor on the op amp. This cuts the gain of the circuit at high frequencies. I used disks marked 22 from a R** S** grab bag, probably about 22 pf, and probably mismarked, knowing R S***. No more oscillation. The feedback resistor is the one that runs from the point of the op amp (output) to the minus input. You might also put in a shorting jack on the input of the amp if it is 1/4 phone or mini phone jack. This shorts the input when the source plug is not attached and prevents picking up RF from from the PC or TV in your room, etc. __________________ Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,SP2-XT's, T-300 HF Projs, Steinway console, Herald RA88a mixer, Wurlitzer 4500 Last edited by indianajo; 27th December 2010 at 05:32 PM.*

15th January 2011, 05:16 PM	#4
whats gan on diyAudio Member Join Date: Oct 2006	p170 thank you for the advice i will try that i bought a single version of the lm4562 and it sounds to my ears as good as the ad 797 and no problems

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28th December 2010, 01:00 AM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	you also want power supply decoupling capacitors as close to the opamps as possible. 100nF from each rail to ground, and 100uF between rails. Honestly, in a circuit like this, you're probably not getting much over using NE5532's

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