DoZ cooling

[starbase218]

Hi,

A while ago, I started my first audio DIY-project, to build a Death of Zen power amp. I completed the power supply, boards and attached some heatsinks, so I was ready to put it in an enclosure.

After asking at a number of local hifi-shops, I got an old Kenwood KA-601 amp that didn't work anymore, to use as an enclosure. A pic of the amp can be found at http://aa-audio.dk/224.jpg.

For heatsinks, I bought four Fischer SK34's (specs can be found here). They're rated at 0.75 K/W. Since I use two of them per channel, I figured I'd get 0,375 K/W per channel. Since Rode says anything under 0.5 K/W per channel should do, I thought this would provide more than enough cooling capacity. In addition, I use four output transistors per channel instead of two, in order to reduce the thermal resistance (from junction to sink) even further.

The problem is that, during tests, the quiescent current doesn't seem to stabilize. I placed all heatsinks on pieces of wood, in such a way that one side of the sinks rests on the wood. Nevertheless, the quiescent current rises continuously, although very slowly. I think this is because the sinks don't provide enough cooling (temperature increases => resistance decreases =>current increases => temperature increases)

I was planning to mount the heatsinks to both sides of the enclosure, so that the fins are vertical. That way, the enclosure (which is made out of iron) will provide some additional cooling (since the sinks will be able to pass some of the heat to the case). The question is, will it be enough? I was already quite surprised to see that, in my test setup, the sinks didn't seem to provide enough cooling to the amp. So I'm very hesitant to mount them this way. I could also mount them horizontally, and place a fan on each side of the amp, in between the sinks. This would probably take care of the cooling problem, but I don't like the idea of hearing the noise of two fans while I'm listening to music.

Any thoughts?

[paulb]

I have some Doz boards that I will use someday...

Are you sure you have good thermal conduction between the transistors and the heat sinks? That's the problem I had when Istarted playing with the boards; one transistor was getting much hotter than the others. Check if there is a significant temperature difference between the transistor case and the heatsink.

[starbase218]

I don't think the case-to-sink thermal resistance is a problem. I can still touch the transistors with my hands while the heatsinks are at about 42C.

[destroyer X]

Put fans on it, inside, under, over...the way you can.... reduce the voltage to reduce speed and noise.

I like this amplifier very much, it is one of the best i have heard, but you can cook some egg or prepare a good coffee with it too.... i do not like this hot...mine works warm....have four fans in a wind tunnel construction.

This way i already post in forum....
"Gonne with the wind...the heat!"

regards,

Carlos

[paulb]

Well, maybe you do need a fan or just reduce the quiescent current. Are you using the zener-stabilized option for the DoZ?
I think with a Class-A design you're faced with either modifying the heatsink / enclosure arrangement for a given voltage / current, or using a given enclosure arrangement and modifying the current so it doesn't heat up too much.
Let us know how it all turns out...
By the way, what are you using for output transistors?

[starbase218]

Originally posted by destroyer X
Put fans on it, inside, under, over...the way you can.... reduce the voltage to reduce speed and noise.

I like this amplifier very much, it is one of the best i have heard, but you can cook some egg or prepare a good coffee with it too.... i do not like this hot...mine works warm....have four fans in a wind tunnel construction.

This way i already post in forum....
"Gonne with the wind...the heat!"

regards,

Carlos

What fans do you use? I have a Spire 8cm fan (intended to cool computers), but it's ball-bearing so it isn't very quiet (although, at lower speeds, it's a lot better).

Originally posted by paulb
Well, maybe you do need a fan or just reduce the quiescent current. Are you using the zener-stabilized option for the DoZ?

Not yet, but I'm going to add this (needed to build the amp and check the voltage first).

I think with a Class-A design you're faced with either modifying the heatsink / enclosure arrangement for a given voltage / current, or using a given enclosure arrangement and modifying the current so it doesn't heat up too much.
Let us know how it all turns out...
By the way, what are you using for output transistors?

2N3055. After I bought them, I read that MJ15003s sound a lot better with this amp... so I intend to replace them (after I get the amp stable).

[phs]

hey starbase218

im also about to build the DoZ at the moment, everything is in parts and drilled, just have to get onto the next stage!
i was going to use 0.78C/W heatsinks per channel, but now im worried

few questions:

did you use rods boards?

are you using metal package 2N3055s? TO-3s? rod says plastic ones will not keep under that amount of heat generated!

have you added the zenner diodes to stabilise shifts in quiescent current? these are ESSENTIAL!

ta
Paul

[starbase218]

Originally posted by phs
hey starbase218

im also about to build the DoZ at the moment, everything is in parts and drilled, just have to get onto the next stage!
i was going to use 0.78C/W heatsinks per channel, but now im worried

few questions:

did you use rods boards?

No, I use universal (epoxy) grid boards. Perhaps not the best solution wrt sound quality, but the amp requires very few components and the boards allow me to modify the circuits very easily. Besides, I made sure the signal paths are very short.

are you using metal package 2N3055s? TO-3s? rod says plastic ones will not keep under that amount of heat generated!

I use all-metal TO-3s .

have you added the zenner diodes to stabilise shifts in quiescent current? these are ESSENTIAL!

ta
Paul

No (see my previous reply). But if it makes that much of a difference, I will measure the temp and current again after I applied the zener(s).

Btw, I found that the Kenwood amp has a big transformer with two seperated secondary windings. So I salvaged that one. Power usage of the Kenwood is rated at 550 watt, so I think it might be possible to use that transformer for the DoZ. Still have to measure secondary voltages, though.

[cunningham]

The link to the scematic (if that is what the link is) doesn't work. However, one point that hasn't been discussed here is temperature coefficient. Does this circuit have a bias servo transistor? If so it should be mounted with outputs. BJT's require less Vbe bias to turn on as it is heated. It sounds like this circuit lacks thermal compensation, or its just misconstruction. Then it could be design but I doubt it. I am not familiar with other peoples ideas...only my own.

[destroyer X]

I can buy them here, 6 for a dollar, and someone in very good condition....have to put the hand inside the basket and have the ones the luck gave to you... sometimes one or two burned...the rule is not to test, this way so cheap...because some of them do not work anymore.

But spending 5 dollares, you can have a good stock after junk the burned ones.

I use two, one each side...and some cover, wooden cover as a tunnel, the exaust one is outside the case..... or facing the case, to exaust to outside environmnet, despite not making too much noise, i do not like things completely cold.... i reduce voltage to feel some warm on the sinks, they are sandwiched with wooden insulators... also i evaluate power feeling the AC voltage with my fingers..... fingermeter..hehe.

regards,

Carlos