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Old 1st June 2012, 12:04 AM   #1
Bando is offline Bando  Canada
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Default New to DIY audio, and need some help

Hello,

I am having my first go at building a Class A solid state amp and it seems logically I am having issues. I found this design Marc's Hobbies and its the 10-14 watt design.

I have the amp on a bread board. I have followed the design exactly except I did not have any TIP3055 but I had some 2N5885 and 2N3055 kicking around. The spec's looked quite similar. I tried the 2N3055, they arc'ed in a second. I tried the 2n5885 because the higher heat dissipation and current load than the 2N3055. I was able to get a current of 1.600 amps on my fluke multimetre. Unfortanetly, it was to quiet. I know it needs a pre and I am using an old shure audio M68A.On top of the insufficient volume, the 2N5885 are HOT, I can almost cook off of them. The heat sink I am using is off an old dukane PA amp, it is about 12 inches long, 4 inches wide and the fins are about 4 inches tall. For two transistors I thought this would be enough. Unfortanetly, upon reviewing if I made any mistakes I broke one of the MJE340.

Sorry for the novel but any ideas,
As well since I am going to have to buy new transistors, would newer more capable transistors be better?

Thank you for your time,

James
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Old 1st June 2012, 02:23 AM   #2
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Default Marc's class A

The TIP3055s are lower rated for dissipation than the 2N3055s or 2N5885 which should give much better cooling, as long as you have good
sinks, mica washers, bushes and suitable thermal grease (non conductive) applied. Otherwise, it's shorts and/or hot transistors indeed.

Sure, 1.6 amps at ~35V will be hot in any case but your sink seems adequate for a stereo pair if the fins are aligned vertical or fan cooled.
Each channel dissipates ~55W or 110W for the single sink and a comfortable max. temp. might be 50C. On a 25C day, that's a heatsink
rating of 0.23/W. Big, but probably OK with your sink.

My sympathies trying to get audio out of a MM pickup without a preamp of any kind. You're out by a factor of ~100 and the impedance
mismatch may squash it like a bug.
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Last edited by Ian Finch; 1st June 2012 at 02:26 AM.
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Old 1st June 2012, 11:50 AM   #3
Bando is offline Bando  Canada
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Thank you for your quick reply,

The Shure M68A has a RCA aux. input and aux. high level output with a pot. So riding the amp and using the pre as volume control. The manual I am able to find states the following, I read it right:

Frequency Respone:
Flat + or - 3db 40 - 20,000hz
Aux. Gain at 1,000hz
50mv to 227mv +13db

As stated in the pdf below.

http://media.musicalplanet.com/pdf/SHU184.PDF

Is this pre no good for this amp?

Would you be able to explain the impedance mismatch?
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Old 1st June 2012, 12:45 PM   #4
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My, that's an oldschool preamp. First off you can be sure to lose 10 dB of that gain due to the power amp's 2k2 input impedance, as output impedance is 4k7. Second, there are two level controls, for AUX and for main volume. Since the power amp has a more than healthy gain of almost 50 dB (for a total of 53 dB in your setup), you should be getting ample volume anyway - assuming everything has been equipped correctly. You are using some kind of high level source, right?

Make sure the remaining transistors are still healthy by performing a quick diode test check (each transistor has two pn junctions and can thus be tested accordingly).

Quiescent current setting on this power amp design is overly primitive. Start out with the adjustment pot in the wrong position, and poof!, these were your power transistors. Not to mention that thermal stability is likely to be abysmal. I'd suggest adding the components for a full-fledged current source. Wait a microsecond...
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Old 1st June 2012, 01:52 PM   #5
Bando is offline Bando  Canada
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Quote:
Originally Posted by sgrossklass View Post
My, that's an oldschool preamp. First off you can be sure to lose 10 dB of that gain due to the power amp's 2k2 input impedance, as output impedance is 4k7. Second, there are two level controls, for AUX and for main volume. Since the power amp has a more than healthy gain of almost 50 dB (for a total of 53 dB in your setup), you should be getting ample volume anyway - assuming everything has been equipped correctly. You are using some kind of high level source, right?
If I understand correctly, if the 2.2K resistor was something lower such as a 500 or even 150. I would not lose as much db's. As well I am using a DVD player, summed to mono.

Quote:
Originally Posted by sgrossklass View Post
Make sure the remaining transistors are still healthy by performing a quick diode test check (each transistor has two pn junctions and can thus be tested accordingly).
I have done this and all except one MJE340 is good, going to buy another one today. Is there a better transistor than MJE340 for this use?

Quote:
Originally Posted by sgrossklass View Post
Quiescent current setting on this power amp design is overly primitive. Start out with the adjustment pot in the wrong position, and poof!, these were your power transistors. Not to mention that thermal stability is likely to be abysmal. I'd suggest adding the components for a full-fledged current source. Wait a microsecond...
From the responses that this amp design is not that great. What you you suggest?

Looking for a Class A transistor amp ( due to simplicity ), to drive my FE83En's.
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Old 1st June 2012, 02:04 PM   #6
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Et voilą, the amp with a current source:
Click the image to open in full size.
Distortion even got a shade better, even though it still is nothing much to write home about (not like anyone expected that).

You can still add an electrolytic in parallel to D2-D3-R8 if you want, like 100µ or so. Shouldn't make a huge difference though.
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Old 1st June 2012, 02:15 PM   #7
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Quote:
Originally Posted by Bando View Post
From the responses that this amp design is not that great. What you you suggest?

Looking for a Class A transistor amp ( due to simplicity ), to drive my FE83En's.
Which are 83 mm, 8 ohm wideband drivers with Pmax of 5 W. So you basically need a few watts of clean output into 8 ohms.

If you want simplicity, I'd just look out for an old soundcard that has an LM1876 on it (some did), and build a gainclone with that. (Other similar ICs are available, up to LM4766 and LM4780.) OK, you'll need a center-tapped xfmr, but it won't need to be a huge one.
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Old 1st June 2012, 04:56 PM   #8
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Well, it's a pretty poor amp design... but I would start by lowering the B+ voltage which will put the power lower, but keep the devices from cooking... you can inch the B+ up once you get it to run.

You might look at the F5 design that is on here, and the JLH class A design threads, these are proven and good sounding low power class A amps.

I presume you want to DIY build it for yourself?
In which case, proceed, expect blown up parts and try to get urself some basic test gear. I'd look on ebay for a dirt cheap 'scope if you don't already have one. You also must have a DVM. I would get a small variable autotransformer ("variac") as well, so that you can power up without smoking parts. Pays for itself pretty fast. That should have a 0-15amp AC ammeter put in series with the DUT (device under test) which is the AC line cord from the amp... that way when you start to turn up the voltage, and the ammeter starts to show WAY too much current, you stop before the devices fry...

There are some other proven class A designs that can be found in threads here. Some have PCBs available. There are also PCBs for some of those designs that are especially popular, and other amps, and minor variations thereof to be had online, including ebay...

At least with a documented and proven design, you can try to follow along the lines of what other folks have already done and shown how to do...

This is a good enough first project to get ur feet wet, the aim should be to make it work, and not blow it up. Then build an amp that sounds good!

Btw there are a variety of means by which you can drop the B+ and still use the same transformer at least for testing...

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Old 2nd June 2012, 01:50 AM   #9
Bando is offline Bando  Canada
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All I wanted was an amp to "get my feet wet". I have a DVM and looking for a scope. Variac will come soon.

Thank you for all the help provided.

James
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Old 3rd June 2012, 01:50 PM   #10
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Ur doing fine!

Everyone has let the magic smoke out, many times.
Even those of us who apparently have lots of experience!

Go for it!

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