Cap suggestions for B&K AV5000 amp

drumbum

Super Member
Need ten 10uf 25v np radial
and
Ten 100uf 25v polar radial.
Both 85c

Have already replaced Rubycon 100uf 100v 85c w/ Nichicon 105c (grade? uhm..they are brown).

Amp runs hot, so would like to step up to 105c.

These are in audio chain.

What would you suggest? Need more info?


Thanks for any help.
 
What vintage is the AV5000? Can't be one of the new generation rebadged ATIs with caps going bad already.
 
The small value caps originally used are NIC brand.
NRSA and NNR series.

Anyone have an opinion on NIC lytic capacitors?
 
Does it matter the grade and brand of caps in amp stage as opposed to preamp?

Will just any ole cap suffice?
 
Good 105 caps with low z are Panasonic FM/FC, Nichicon PW (and many more there's way too many series). But those should be fine, just maybe not top-line specs which doesn't really matter. Plenty of cap shoot-out threads on here.
 
Amp Has been DBTested and all channels up and working. Been testing with some Sandy Duncan's Eye, My Dad Is Dead, PJ Harvey, and some Stan Getz/Charlie Byrd.

These amps are known to run rather warm. Don't think anyone has ever worked on it previously.
I have measured a temp of 180* directly on heat sink and 143* on case.

Anyone have any idea what the idle current should look like? I have the service manual, but no mention of adjustments; basically just schematics and BOM.

I have a buddy with one and it runs room temp IN A CABINET! That one has been dicked with and I know it isn't right. I heard it hooked to some Altec Santiagos and I would have expected it to be able to run me out the room, but nope; a touch anemic.

Anyone gots idears?
 
Word I've gotten from techs who should know is spec for "bias" current to each amp board (no input, no output) was 100ma per pair of output MOSFETs plus 50mA for the rest of the circuit. However, they will still run—and do so a LOT cooler—on half that although many of our golden-eared brethren are of the opinion low bias negatively impacts SQ. Some even (I think at the behest of the head dweeb at Stereophile) recommend juicing the bias higher than factory for the best SQ at the cost of more heat and shorter component lifetime.

Anyway, I'd advise you try 75mA per pair on one channel (so if 5 pairs, 5x75ma+50ma=425ma), 100mA on another, and 50mA on a third. Compare each channel against the others for SQ and keep an eye (or a thermal probe) on the heatsinks. Please report back what you find out and how you decide to go. Good luck.
 
Word I've gotten from techs who should know is spec for "bias" current to each amp board (no input, no output) was 100ma per pair of output MOSFETs plus 50mA for the rest of the circuit. However, they will still run—and do so a LOT cooler—on half that although many of our golden-eared brethren are of the opinion low bias negatively impacts SQ. Some even (I think at the behest of the head dweeb at Stereophile) recommend juicing the bias higher than factory for the best SQ at the cost of more heat and shorter component lifetime.

Anyway, I'd advise you try 75mA per pair on one channel (so if 5 pairs, 5x75ma+50ma=425ma), 100mA on another, and 50mA on a third. Compare each channel against the others for SQ and keep an eye (or a thermal probe) on the heatsinks. Please report back what you find out and how you decide to go. Good luck.

Specific to B&K AV5000 correct?

Threw the lid on it and have continued testing. Running ch. 1 & 2 (ch. amps next together on the sink), I have measured 153* on the steel case/lid above sink. Ch. 4 & 5 (split on sink) side of amp, unused measure 116*. Concern?
 
B&K amps have been basically the same for decades. Here's a bias setting guide from the horse's mouth (and a counter to the Stereophile recommendation). You can check out the the architecture of the amps mentioned to determine whether the 100mA/pair + 50ma "rule" applies. If you require more definitive direction, I suggest you call Steve over at Stargate Electronics in Long Island NY or John Hillig at Musical Concepts/Musical Design in Saint Peters MO.

On your "experiment" what board currents are you running the various channels at? Also, how many MOSFET pairs do you have per channel? As far as specific or absolute temps, I have no opinion one way or the other. I only distinguish between relative temps—hotter or cooler—given higher or lower bias. If SQ is the same across a REASONABLE bias current range, then I'd tend to go lower current and cooler operation rather than higher and hotter. Simple.
 

Attachments

  • BKComponentsBiasSettingGuide.pdf
    72.8 KB · Views: 39
B&K amps have been basically the same for decades. Here's a bias setting guide from the horse's mouth (and a counter to the Stereophile recommendation). You can check out the the architecture of the amps mentioned to determine whether the 100mA/pair + 50ma "rule" applies. If you require more definitive direction, I suggest you call Steve over at Stargate Electronics in Long Island NY or John Hillig at Musical Concepts/Musical Design in Saint Peters MO.

On your "experiment" what board currents are you running the various channels at? Also, how many MOSFET pairs do you have per channel? As far as specific or absolute temps, I have no opinion one way or the other. I only distinguish between relative temps—hotter or cooler—given higher or lower bias. If SQ is the same across a REASONABLE bias current range, then I'd tend to go lower current and cooler operation rather than higher and hotter. Simple.
Thanks for the bias setting guide!

Dealing with two pairs per channel.
 
Just be careful you don't forget you're in circuit measuring the "killing" current and go spinning the dial...


Something to be said for measuring voltage across a resistor as an analog for bias current, IMO.
 
Just be careful you don't forget you're in circuit measuring the "killing" current and go spinning the dial...


Something to be said for measuring voltage across a resistor as an analog for bias current, IMO.
Well, backed it all down. No load idle temps on heat sink-3 ch sink 125* 2ch sink 118*.

awillia6, tell me if this sounds correct.
I measured across the emitter resistors of one pair and set the current @ 50mv. Should I average the the two pairs per channel? If I pull a fuse and go across I should get around 100ma, right?

Even 50mv seems high seeing as how i just set my yamaha p2250 to 12mv per spec.

Am I to understand Mosfets don't "need" much idle current if the pairs are closely "matched pairs"? Something about making sure they are both "on" at low listening levels...

My brain feels hot.
 
From all I've heard from techs who work on B&K amps on a regular basis, standard idle current for B&K (as well as other MOSFET amps) ranges from 100-150mA per push/pull pair. They can be run lower (down to 50-75mA) with some (though arguably none) level of SQ degradation, lower overall thermal loads, and unknown effects on circuit stability depending on design. The method of establishing B&K board current at idle (encompasses ALL output pairs plus other circuity on a given power feed) is to pull one of the two board fuses, place an ammeter between the fuse end clips, and adjust the appropriate variable resistor to give a board current reading of n x 100mA + 50mA where n is the number of parallel output pairs per board and the 50mA is to drive the other circuit components on the same board. I believe this current level runs the B&K amps too hot, shortening the life of its components (especially drying out electrolyte in capacitors).

On your matching question, MOSFETs do not have near the variability of BJTs so matching is inherent in the manufacturing process and can be disregarded in selecting individual pieces.

For a highly technical discussion of all things MOSFET, suggest you consult Cordell's Designing Audio Power Amplifiers, McGraw-Hill, 2011.
 
Last edited:
Back
Top Bottom