B251 Fan (Lüfter)

[MilanMilosevic]

Re: B251 Fan (Lüfter)

I guess I will build something by myself. Here is the schematics of a simple NTC-driven fan controller. I guess I could also build a PWM controller but this one is simple enough. I made the drawing and the whole assembly can fit onto a 2,5 cm x 4 cm PCB. I would set the controller to kick in when the temperature on the heat sink reaches 50 C.
I already have a silent fan inside the amplifier. It does a good job in sucking the air across the cooling fins. However, it doesn't seem to cool down the driver transistors (I have heat sinks mounted on all of them) so while the top right side is cold to touch, the left side stays considerably hot. I think the only way to solve this would be to place a large diameter (18cm or bigger), slow rotation fan on top of B251 but I would somehow like to avoid that.

[MilanMilosevic]

Re: B251 Fan (Lüfter)

According to the diagram in my previous post, I have built a simple fan controller (the diagram has a fault because the symbol is for N-FET and this schematics requires a P FET transistor). The whole assembly fits on 2,5 cm wide protoboard and costs less than 5 Euros to make. The controller is now in the testing phase: the PCB is positioned next to uP PCB and is fed 11V from the power supply. NTC thermistor is attached to the copper cooling pipe with epoxy resin, in the narrow space between the power amplifier PCB and the first cooling fin - this is the closest I could get without bolting the thermistor directly to output transistors. I am using Noctua NF-R8, very smooth and silent fan, the plan is to have two connected in parallel and blowing at half speed but more evenly across the heatsink - note that on controller PCB there are two fan connectors. The controller is set to switch the fan at 35 degrees C and measurement point. This temperature is purely empirical: the amplifier reaches it after approximately 15 minutes and gets more than warm. I have no idea how high the temperature gets at transistors. The NTC will drop resistance rather quickly and is not ideal regulator as more sophisticated PWM circuits but the curve is still good enough to give gradual RPM regulation. This whole system, however, does not seem to solve lots of heat of the left side of amplifier, cause by the stabilization heatsink and driver transistors. The lid feels cooler but temperatures of driver transistors still remain high even with active cooling.
I am thinking about using the R531 NTC on power amplifier as the measurement point, use a different thermistor and connect it directly to the controller. What would the reference temperature be in that case?
Any advice for a different measurement point and setting a different trigger temperature is welcome.

[MilanMilosevic]

Re: B251 Fan (Lüfter)

After several months of experimenting with various positions of the cooling fan and measuring temperatures and effects, I would like to share with you my findings.

The first conclusion: if an effective thermal design hasn't been originally implemented by the manufacturer, it is very difficult, if not impossible, to make a satisfactory solution afterwards.

Here is what I tried:

1. Cooling fan on the back of the amplifier - centered to the cooling fins - sucking air out
I tried two temperature settings: 40 and 45 degrees at output transistors. The circuit and the fan did a good job in keeping the right half of the amplifier away from hot. The 40 degrees setting gave somewhat "colder" sound, so obviously that was too much below the operation points.
Problems: a) After 3 months, I found the inside of the amplifier covered with layer of dust, b) the left half of the amplifier with power supply, bias and driver transistors (where the main problem is) was still too hot, going over 80 degrees (Q117/Q317)

2. Cooling fan on top of amplifier - above the bias PCB - sucking air out
This position was more effective for cooling and I set the target temperature on Q117/Q317 at 70 degrees.
Problem: it was impossible to obtain uniform cooling of +/- driver transistor pairs on same channel, so the DC offset began drifting (could also be because of drifting quiescent current)

3. Cooling fan on top of amplifier - above the bias PCB - blowing air in
I reversed the fan direction to try the "positive pressure" principle and the cooler achieved working temperature faster than when sucking the air out
Problem: this actually pushes the heat back in the amplifier which then travels through the internals and heats other part. As the result, it also gave temperature rise to the output transistors way over 50 degrees and the whole right half of the amplifier became too hot

4. Cooling fan on top of the amplifier - blowing laterally, across the cover, from left to right side
More or less the same effectiveness as 3, with the added benefit of also blowing across the cooling fins
Problem: due to air stream, the driver transistors are either too hot or too cool; in the second case, the quiescent current goes too much up and heats up the output transistors.

I disconnected the cooling fan and put the whole "project" aside for the time being and am thinking now what to do next because none of the methods I tried gave satisfactory results. Perhaps someone has an advice to share?

[Wolfgang/Wien]

AW: B251 Fan (Lüfter)

Perhaps someone has an advice to share?

Buy a different kind of amp an use this one for the kitchen.



Other idea:
Use your B251 WITHOUT any additional fan, as thousends of other people do...



no more ideas



regards from vienna

[0300_infanterie]

AW: B251 Fan (Lüfter)

That´s my first thought ... is there a real need for that? Also to the ext. 12V-Solution shown for the B750 ...
Were there anytime such problems known?

[borchee]

AW: B251 Fan (Lüfter)

... is there a real need for that?...

It seems the B251 has the same issues as the B285 - due to extensive heat, solder points can unsolder themselves and PCB tracks/eyes can be lifted and/or get broken. Never heard about the B750 having this problem though...well, at least none of mine is affected.

Milane, after reading your posts more carefully, i see that the original fan, being mounted behind the heat pipe, isn't the way to go.
Equiping the transistors with radiators, if not already done, can't be a bad idea...perhaps not necessary as done in http://new-hifi-classic.de/forum/index.php?topic=6012.0 - the man used a fan also - but changing the named elements is a revox modification/improvement regarding the heat problem, if i understand correctly.

How about, for the love of experiments, driving the thing without bottom, top and side panels?

A B251 with two external fans, altough no details: http://www.hifimuseum.de/revox-b251.html

Below the original cooling fan.

Regards
Borut

[MilanMilosevic]

Re: B251 Fan (Lüfter)

Zdravo Borute,

thank you for your response and the links.
I didn't know about B285 but yes, B251 does have a bad thermal design: horizontal PCB layout and no natural air flow. The power amp sounds great but is badly designed, because the 8 driver transistors which run at 80+ degrees are attached to PCB instead on some heatsink.
The back cooling fan, either factory one or DIY, is not very useful. It cools the output transistors which rarely warm up over 50 degrees, which is not that high and is not where the problem is. Moreover, that fan has absolutely no effect on driver transistors and as the downside, will fill the amplifier up with a layer of dust in a matter of months.
Putting one or two fans on top, above the problematic transistors will solve the heat problem by sucking the hot air out, but will cause others: because driver transistors are sensing the temperature, cooling them down by even a couple of degrees will cause quiescent current to rise, thus giving rise to the temperature of output transistors as well. Also, because it is impossible to place fans in such a way that they equally cool + and - drivers, disbalances will occur, and so will disbalances between channels due to temperature unevenness. Trust me, I have tried, experimented and measured every solution and they are simply unacceptable.
I did mount beefy heat sinks on all 8 transistors and that is all I can recommend. Another solution would be to mount these transistors to a common heat sink and away from PCB. So, in any case, passive cooling is my advice.
Finally, I did change the PSU voltage from 220 to 240 V last night and after checking all regulated and unregulated voltages OK, I can also confirm that the temperatures are about 5 degrees lower than before.

[borchee]

AW: Re: B251 Fan (Lüfter)

Zdravo Milane,

...Trust me...

i do, without a doubt.

...Finally, I did change the PSU voltage from 220 to 240 V last night and after checking all regulated and unregulated voltages OK, I can also confirm that the temperatures are about 5 degrees lower than before.

OK, that confirms what i read, altough i couldn't find a statement, beside of your's now, about the degree of "colling".
What about the modifications as in the link and service manual (comparing them demands too much of me, so i'm not absolutely sure, if they are one and the same), are you considering them also?