Solid State DIY

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Other DIY amp plans

Here are a few other amps: 20W

By the way, how far along are you on the a40 design? I've been spending the past few weeks tracking down the necessary parts. I still cannot find the 1W 5% wirewound resistor, and am having trouble finding a suitable toroidal transformer. Have you found your transformer yet?

If you are interested, I'll post the URLs for the remainder of the a40 components I have found...


[Edited by Eric on 12-06-2000 at 01:53 PM]

Yes I am interested in the links you have for parts. Digikey has the 1 Watt wirewounds.
The transformer I will be ordering from Plitron @ I have not yet decided to use 1 or 2 transformers yet. Today I will start etching the boards (or at least that was the plan) so really not very far along at all yet. I have the transistor or replacements so I may need to do some tweaking but they should do the trick. The heatsink I'm going to try and talk my brother into making (hes a machinist) Keep me (us) updated on your progress.

Thanks for the links!
Plitron has transformers that will work no need to custom order. I sent Mr. Pass an email questioning him on the transformers. All you need is 22 volt secondaries (22 times 1.4142 yields about 30 volts) and he indicated the A40 draws about 100 Watts per channel. So double the wattage for saftey (200 watts) that equates to a 400 VA (both channels) and you can always make it larger.

My DigiKey catolog maybe different I am from Canada and they have seperate catologs, but they should have them. I show them having Yageo wirewounds 5 and 10 Watts with .68 ohms. I wouldn't worry to much about finding 1 watt a 5 watt should work just as well.

Etching isn't as hard as it may sound. I use the toner transfer method. Have tried the photo method but it failed terribly. and are good sites. Although the price of the boards from Old Colony sounds good.

Mr Pass addresses not using the same devices he did in the article. I think (without looking at schematics) R11 may need to be tweaked to achieve the correct bias.

Keep in touch happy hunting!
a few more questions

OK, found the 5w 5% wirewounds - I wasn't sure if these would adequately substitute for the 1w resistors. I've updated my links above for the parts.

Next, I think I'm starting to figure out the rating scheme for transformers. First, a question about 22 volt secondaries: Why are you multiplying 22v by the square root of 2 to obtain 33 volts? Where does the sqrt(2)come from? From the bridge? Going back to Nelson's design, it appears that the specified transformer (118v primary, two 44v secondaries, 8A) is WAY more (704va primary!) than this design actually needs. Is this correct?

It looks like (using your logic) I could use a (slightly larger than needed) 400VA primary with two 25VA secondaries - should be more than enough. Is this correct?

Finally, I remember seeing Nelson's warning about substituting parts but the original output transistors are no longer available... Other people seem to have used the Motorolas without any trouble.

Thanks for your help!

[Edited by Eric on 12-07-2000 at 11:06 AM]
AC Theory

Its been a long time since I used AC theory so if anyone else can explain it better please do so. But 1.4142 is used to find the peak voltage. A DVM on AC volts typically takes an RMS value or average. (AC is a sine wave) So for 115 VAC the incoming peak would be 162.6 VAC. So 22 volts would give 31.1 Vac peak. If you really want to get fancy in the bride there will be 2 volt drops because of the diodes. For theory say 0.7 volts per diode for 1.4 Volts. So really your left with 31.11 - 1.4 = 29.71 VDC. In the real world it may end up higher transformers are not perfect.

HOpe this helped
toroids and power rating

Thanks for the explanation of toroid power ratings. I was looking around some more and found a good section in the description of Heatsinking for the Pass Labs A75 power amp: It relates heat to power in the context of transformer and heatsinking selection.

Did you get a price quote from Plintron about the transformer cost? It looks like there is an appropriate transformer on the sealelectronics home page: It looks like transformer #3 will do nicely, and have plenty of power left over for headroom. I was wondering how the prices compare (SealElectronics gets transformers from Plinton)...

Transformer 3 as I see it has 40 volt secondaries. This is much to high. 40 volts * 1.142 = 56 volts for the A-40 you want about 30 volts. You need a transformer with about 22-24 volt secondaries with a VA rating of at least 400(VA).
If you look in the A-40 article under Final Tests you will see N. Pass is expecting +- 32 volts. part # 757015201 has 400 va rating and 22 volt secondaries cost $53.00 US or $73.00 Canadian. Once again you may want to go with a higher VA rating than 400VA. Or use two seperate transformers with ratings of 200VA or higher. I emailed a fellow from Australia and he is using 2 - 350VA transformers.
I know you haven't started construction yet but when you do be very careful! The transformer is rated for close to 10 Amps. If you are inexperienced with high voltage I would enlist the help of someone who is. The power supply caps will knock your butt if your lucky! Saftey first!

Once again hope this helped
A little confused?

I think I have found the source of our differences in the voltage specification for the secondaries:

The specs in the design document for the amp call for a transformer with "two 44v, ct, 8A secondaries". Is this an error? Based upon what is said later on in the article (Final Testing, as you point out) it seems that the transformer should be a 44v (total) secondary arranged as 22v + 22v in parallel. Is this correct?

This then keeps voltage at ~30v for each channel (22*1.4142).

Thanks for the warning on the power caps - I'm aware that they hold a significant charge for quite some time. I'm thinking of bypassing the large caps with smaller ones for discharge when the amp is shut off...
Hows the progress on the A-40?

Hi Doktor,

Just checking in to see if you've been able to make any progress on the a-40 amp... I'm still looking up suppliers for various parts and have updated my list above with a few things I've found. I probably won't start building for a little while, but I've slowly started collecting parts.

Eventually, I'd like to build a set of Northcreek Rythm Revelator speakers to go with the new amp - but now I'm just being ambitious!!

If you've gotten there yet, I'd be curious to know what value of resistor you are using for R11 to set the bias with the motorola output devices.

Sorry it took so long for the update been away for business. Well I have the PC boards etched and most components mounted still looking for 220uF tantalum caps. Picked up a transformer from a surplus place. Basically the heatsinks are the one thing holding me back because the enclosure is going to be impacked greatly by them. Been looking at one from C & S sales its like 17 * 7 * 3 very large. I have some pictures I plan to post on my web site in the future so stay posted.
Heat Sinks

I found some Wakefield 423A heatsinks (0.67c/w) used/surplus from Marlin P Jones & Assoc for $7 each (new retail is $20). I bought 8 of them, one for each output device. They are about 5.5" tall, and 4.75" wide. So I'll put 4 on each side of the chasis. I'm envisioning a chasis size of 10-12" wide, 20" deep and 6" high. With heatsinks on both sides, overall width is likely to be 15-17". I don't know if they have any more of these a MPJA, they don't list them on the web page since I bought min...
Original PCB

Yes, I have the original PCB, but beware, there is an error on it. One of the traces needs to be cut, and another trace needs to be added. When you order the boards from AudioXpress, they have a note to this effect.

If you mount the new output devices directly on the PCB, how are you arranging heatsinking for them?
Hi Eric.
Since the TIP transistors are in a TO218 house I've placed them at the one side, right at the edge, of the PCB, so they can be screwed right on to the heatsink.
No more need for often too long wires between the PCB and the transistors, and furthermore, no need to drill all them ...... holes needed for the TO3.
Pretty Clever

Looks like that would certainly reduce the need for external wiring. Fortunately, my heatsinks came pre-drilled (got 'em surplus!) so the only thing I need to deal with is running wires to each of them (still have to drill thru chasis). I think that the MJ's have a little higher temp and current ratings - I was comtemplating running the bias a little higher for increased power into a 4 ohm load...

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