so it has bias trimpot, amp is hot around 75C degrees under 4ohm stable load.
I mean speakers with stable 4ohm impendance.
i measured 5mv across 0.68ohm emmiter resistor.
I mean speakers with stable 4ohm impendance.
i measured 5mv across 0.68ohm emmiter resistor.
5mV, if you measured as you said, is only 7.5mA per output - heatsink should only be warm to the touch. Sounds like you have a problem, or you measured incorrectly.
Make sure you are measuring like this - input shorted, no speakers attached and after it has sat powered on but at idle for 10 minutes. Measure across the emitter resistors and take that measurement to calculate bias in mA (i.e. 5mV/0.68r= 7.35mA).
I would not run it at 75C for very long, or you are asking for a meltdown.
Make sure you are measuring like this - input shorted, no speakers attached and after it has sat powered on but at idle for 10 minutes. Measure across the emitter resistors and take that measurement to calculate bias in mA (i.e. 5mV/0.68r= 7.35mA).
I would not run it at 75C for very long, or you are asking for a meltdown.
I measure it exactly as you said, i have 5.3mv across emmiter resistor 0.68ohm.
In idle this around 45C degrees after hours this does not change at all.
I tried to raise it to 12mv, and heatsink become 46C degrees and stay the same for hours. I do not playing music, just to test.
When i pushing - after 20 mins or so it become around 75C and wery fast cool down after i reduce the volume.
Its with stock 5.3mv drop.
Also Krell give me schematics, but not service manual. I dont know wich bias it might be.
I can send scheme via email if somebody can take a look and find correct bias voltage.
In idle this around 45C degrees after hours this does not change at all.
I tried to raise it to 12mv, and heatsink become 46C degrees and stay the same for hours. I do not playing music, just to test.
When i pushing - after 20 mins or so it become around 75C and wery fast cool down after i reduce the volume.
Its with stock 5.3mv drop.
Also Krell give me schematics, but not service manual. I dont know wich bias it might be.
I can send scheme via email if somebody can take a look and find correct bias voltage.
If it reaches 75C at 5mV, then leave it alone.
I certainly wouldn’t go over 10mV and if you raise it, be sure and check the heatsink temps. I keep any heatsink below 60C max and prefer 50C on my amps for long-life.
Sounds like it is bias right.
I certainly wouldn’t go over 10mV and if you raise it, be sure and check the heatsink temps. I keep any heatsink below 60C max and prefer 50C on my amps for long-life.
Sounds like it is bias right.
It does not.
Based on the 80W idle power from the published specs, the 75v rails and the 20 output devices i would guess at least 60W being dissipated on the heatsink.
Which, if not mistaken gives 40mA bias per transistor or 27mV across each emitter resistor.
Attached is the Schematic I got from the OP:
Running some calcs based on the spreadsheet and the following parameters: (confirms analog_sa's numbers)
40 watts consumption during warm-up ie about 15 watts dissipation per channel (plus 10 in other parts of the amp) requires about 15mV across each 0.68 ohm ER (or 19mA).
Once warmed up, the amp idles at about 80w (about 30w per channel assuming 20 watts in other parts of the amp) and to get to 30w dissipation, the current per device is about 40mA or 27.5mV across the ER. This gives the amp a little less than 1 watt pure class-A operation.
So I'd set it at 15mV and monitor the temperature and voltage drift. Fully warmed it should be about 27.5mV. The sinks are small on this amp, so I'd say it will idle between 45C to 55C based on past experiences. If it goes higher, check each device and make sure they are all about the same temperature and if they are, reduce the bias till you peak to around 50C.
Hope this helps.
Running some calcs based on the spreadsheet and the following parameters: (confirms analog_sa's numbers)
40 watts consumption during warm-up ie about 15 watts dissipation per channel (plus 10 in other parts of the amp) requires about 15mV across each 0.68 ohm ER (or 19mA).
Once warmed up, the amp idles at about 80w (about 30w per channel assuming 20 watts in other parts of the amp) and to get to 30w dissipation, the current per device is about 40mA or 27.5mV across the ER. This gives the amp a little less than 1 watt pure class-A operation.
So I'd set it at 15mV and monitor the temperature and voltage drift. Fully warmed it should be about 27.5mV. The sinks are small on this amp, so I'd say it will idle between 45C to 55C based on past experiences. If it goes higher, check each device and make sure they are all about the same temperature and if they are, reduce the bias till you peak to around 50C.
Hope this helps.
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Bias is set with
1) no signal input (short the input if you can)
2) no speaker connected for the initial run.
If it goes down to 5mV later on with a 4 ohm load... it just means the bias transistor is compensating. (assuming everything is ok in the amp)
I have found Sankens to be quite Robust, I wonder why they stopped manufacturing the A1295 pair that many nice amplifiers had.
Whats interesting here in the KAV-2250 is the use of the Sanken A1860/C4886 pairs as the OP's, driver and pre-drivers. 5 pairs in the Output. These devices have a Vce of 150vdc and Krell maxes out (No margin) the rails at +/-75vdc. To get to the same power as the predecessor (KAV-250) they use a 2.5kVA Toroid vs the 1.8kVA on the KAV-250. The Older 250 had 88v-91v rails and used 5 pairs of 250w metal can ON devices. The newer 2250, seems a lot more fragile in comparison.... but faster devices 50Mhz vs 30Mhz on the older parts.
Whats interesting here in the KAV-2250 is the use of the Sanken A1860/C4886 pairs as the OP's, driver and pre-drivers. 5 pairs in the Output. These devices have a Vce of 150vdc and Krell maxes out (No margin) the rails at +/-75vdc. To get to the same power as the predecessor (KAV-250) they use a 2.5kVA Toroid vs the 1.8kVA on the KAV-250. The Older 250 had 88v-91v rails and used 5 pairs of 250w metal can ON devices. The newer 2250, seems a lot more fragile in comparison.... but faster devices 50Mhz vs 30Mhz on the older parts.
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I set one channel bias to 15mV, and bias remains stable, seems do not changed with temp go high.
I waiting till warmup.
So wery intresting thing happened.
Channel with 5.3mV bias running 3C warmer then channel with 15mV bias..
When they both 5.3mV temps on them identical.
Channel with 5.3mV bias running 3C warmer then channel with 15mV bias..
When they both 5.3mV temps on them identical.
The manufacturers ( Naim ) state it runs in class A for a certain wattage then in class AB if that isn't variable then what is ?
You don't even know who the manufacturer is, but know all sort of things related to the classes 😀
Unlike Krell's well known big amps with sustained plateau bias, the small guys run at fixed bias. This one too. And as every class AB amp it works in class A up to some level. K-amps has been kind enough to even calculate it.
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Can you measure the power drawn from the mains? It is the best indication that the idling is correct.
Can you also confirm that the emitter resistors in the amp are indeed 0.68ohm as indicated on the circuit? It is not unusual that production amps use different emitter resistors and this will of course change the target voltage.