amp/capacitor question

[rrcrain]

My amplifiers are based on the leach Super amp design and are well over 25 years old. Realistically, it might be a wise idea to replace the electrolytic caps now before they start failing. Thankfully, only two caps per channel are in the audio path. One cap, a 220uf NP is in the feedback loop and if I read the manual correctly, sets the gain at 1 (unity gain?). If room permits, is there anything to be gained by replacing this cap with a poly, or is an electrolytic good enough?

All of the other electrolytics in the amp are on the 100 VDC power rails.

[dynaco_dan]

>My amplifiers are based on the leach Super amp design and are

>well over 25 years old. Realistically, it might be a wise idea

>to replace the electrolytic caps now before they start

>failing.

.

Please read below.

.

Thankfully, only two caps per channel are in the

>audio path. One cap, a 220uf NP is in the feedback loop and if

>I read the manual correctly, sets the gain at 1 (unity gain?).

>If room permits, is there anything to be gained by replacing

>this cap with a poly, or is an electrolytic good enough?

.

Please read below.

.

>

>All of the other electrolytics in the amp are on the 100 VDC

>power rails.

.

Please read below.

.

Hi Richard;

I just saw this write-up a few minutes ago.

Leech! YAHOO!!

You have my full attention!!

If you are referring to, W Marshall Leech Jr's, Double Barrel/Super Amp or Low Tim amplifier, from '76 - '80, I would most definitely like to hear more from you about it.

I just received an email reply back from Marshall last week.

I have a small project that I am working on for him.

He has a very nice web site up and running.

He has updated the Super Amp in a few area's.

What type, model and brand elect cap is in your power supply?

Can you take a few dozen (JOKE) photos, the more the merrier at least of the inside and outside of the amp?

There is a 4 mb limit to downloading here.

If you can download in several sessions it is better quality to view, contuing with the LEECH heading.

I was given some photocopies of the Double Barrel amp article back in the early '90's from my big bros.

He built a Leech Low Tim 4 and Leech pre-amp from scratch.

He used military spec parts where available to him.

I had been given a blank gold anodized Dynaco Pat-4 size face plate, by a friend who had owned the stereo service shop I worked at for a few years.

I moved a few years ago and I could not remember where I had placed it.

My big bros gave me some photos of his amp and pre-amp.

There was the face plate, I had given it to him and he used it for the preamp, at least his version 1 preamp.

Here are a few photos of his creations.

The last photo is the blank Dynaco face plate, which was available blank with no printing or holes from Dynaco.

Those red coloured 3 pin gold plated pins transistor sockets are about $10.00 US each now, I believe they are Augat brand.

He would buy the transistors by the bag of hundreds and try to match them as close as possible.

In about 1995 I started with my brothers list of suppliers, looking for the Double Barrel amp parts suppliers.

Most were already out of business.

Even while I was trying to buy from existing local sources some prices had already shot right out of sight.

TO-3 August sockets gold plated pins where one only was $1.75 CDN here surplus.

Mailorder today, almost $12.00 US each from Mouser.

I never was successful finding every thing that I needed.

We really do not have the old surplus electronic stores that you may have in your larger cities, you lucky folks.

I lucked out here once in a while with some parts though.

We will talk more about your amp when you write or post some photos, please.

[dynaco_dan]

Hi again;

I am sending Wally's information back to him next week.

This is an extremely good example of why I ask for the best possible resolution be taken when photographing or scanning something.

The two attachments here are of the same previous circuit boards but the first one is the original, that I have at least, and the second is reduced by emailing to myself and reducing it's size first.

I had asked my big bros to copy the best possible photo, he said I can't print it out that good, so what the heck.

He had already reduced it before sending it to me.

My point will be quite obvious soon, if you try to enlarge these images on your computer.

573 KB versus 66 KB

The first photo will enlarge so that you can read the resistor bands, quite clearly, the second one, well, it sucks by comparison.

You can still see the circuit board.

It takes the same time to scan at a higher quality and save to a CD than a poor copy.

We only have a few opportunities to save this valuable archival information, we may as well do the best we can now.

Now a really good copy will be 14 MB or there about.

This is not to be printed out or emailed.

I mentioned to Wally, of a woman's photo, with her about 3 feet from a camera, I can count the pores on her nose.

We do not have room for a nose collection library here but the photo can be reduced for the library or emailing or just burnt to a cd.

By the way, because the photo was taken at an angle, the right side is clearer than the left side of the left board anyways.

[rrcrain]

Unfortunately, my followup post was lost.

According to Leach, this amp was designed by one of his students who went to work for the company that produced them, and switched all of the PNP and NPN devices from the original design. This is darn near the only difference from the original Leach super amp design.

I still have the schematic for the amp along with the original manual and circuit description. I'l do what I can to scan the schematic and post it. As for circuit board photos, that's just short of impossible given the size of the heat sinks.

This amp can still be occasionally picked up on Ebay and very few people have a clue what this amp really is or can do.

[dynaco_dan]

>Unfortunately, my followup post was lost.

>

>According to Leach, this amp was designed by one of his

>students who went to work for the company that produced them,

>and switched all of the PNP and NPN devices from the original

>design. This is darn near the only difference from the

>original Leach super amp design.

>

>I still have the schematic for the amp along with the original

>manual and circuit description. I'l do what I can to scan the

>schematic and post it. As for circuit board photos, that's

>just short of impossible given the size of the heat sinks.

>

>This amp can still be occasionally picked up on Ebay and very

>few people have a clue what this amp really is or can do.

>

Hi again Richard;

If you can just take some general photos and post them, it would be great.

There has been more that just the transistor changes since new.

The original Double Barrel amp was only a mono amp, capable of around 300 watts RMS per channel.

Since published in 1980 there has been several supplier changes, Motorola has discontinued some of the transistor's and now ON is the manufacturer.

Because of oscillations, some parts were even re-located for safety.

Is this the amplifier that you own?

LSR&D amplifier?

[rrcrain]

Mono? That makes sense given these amps are almost a dual mono design with only a massive power supply in common. The rail voltage is +/- 100 VDC with 5 amp fuses per channel on each rail.

When did Motorola discontinue making the transistors? The last I knew, they were still making them a few years ago.

I can handle general photos and have a few on hand; All I have to do is find them. With luck, I'll find time tonight to upload a few.

Turn on current is incredible. If both amps turn on together, they can occasionally pop a 15 amp breaker.

[dynaco_dan]

>Mono? That makes sense given these amps are almost a dual

>mono design with only a massive power supply in common. The

>rail voltage is +/- 100 VDC with 5 amp fuses per channel on

>each rail.

>

>When did Motorola discontinue making the transistors? The last

>I knew, they were still making them a few years ago.

>

>I can handle general photos and have a few on hand; All I have

>to do is find them. With luck, I'll find time tonight to

>upload a few.

>

>Turn on current is incredible. If both amps turn on together,

>they can occasionally pop a 15 amp breaker.

Hi again Richard;

The original article in Audio magazine was only a mono amp.

You had to read near the end of part 2, I believe, to find mention of that fact.

The power transformer was 38 pounds by itself.

A Dynaco 400 transformer is about 20 - 25 pounds by itself.

When I received a quote for the original spec'd transformer, about 1995, it was going to cost me around $400.00 - $450.00 CDN landed for one.

Depending on the primary windings tap used, you could push the power supply up to 92 volts from 85 with 10 amp fuses being used.

I am guessing here that Motorola sold off the least profitable models and kept the best sellers.

You may be able to pick up some of the original MJ15001 - 4 TO-3 transistors in an electronics surplus section still.

I am always referring to the Audio magazine project here.

An up-date suggested that MJ15003 & 4 could be used instead of MJ15001 - 2, meaning that a better quantity price may have been available at that time as well.

If you look back at the earlier photos, you can see a slow start circuit my big bros added to the amp at the right rear of the enclosure.

The circuit was a Walt Jung designed circuit, I believe.

The delay is adjustable by changing a cap or resistor value or using a pot instead.

I've dabbled with a version I made and I've tried various time delays.

Except for one surplus Augat TO-3 transistor socket, I never came across another for a year or two.

Except one day, I really lucked out with a local surplus store and I found a great number of them at a very, very attractive price, so I bought them all.

The spec'd heatsinks were about $35.00 each.

The spec's caps were Low ESR, which were about $80.00 each so I bought std ESR caps.

With the demise of Heathkit, Dynaco and others in the kit building field, off shore importation of stereo equipment, and LSI, DIY parts have become rarer.

Did you come forward here a year or two ago to say you had a Leech amp?

If not, then another member did.

Looking forward to your photos, Richard.

[rrcrain]

I'm humbled and confused.

I absolutely know I have several digital photos of these amps, inside and out. The amp is a flat black and doesn't photograph for heck, but I have several somewhere. I also have several photos of the circuit boards... and I can not find them. My computer has over 400 gigabytes of storage and I seldom delete anything, let alone photos. Only thing I can think of is the files have an obtuse default name.

In any event, the power supply uses two 13,000uf caps with 100 VDC applied to each. There is a time delay circuit for a soft start. A 3.3 ohm thermister is briefly placed in series with the primary winding to limit startup current.

Each amplifier channel contains 8 TO-3 transistors, four MJ15024 and four MJ15025. One of the changes I seem to remember from the original super amp design is the addition of an offset adjustment in this amp. The feedback circuit consists of a 220 uf NP electrolytic and a 120 ohm carbon resistor. The only other caps in the audio path is C101, which is a 10uf NP electrolytic. I changed these out earlier this year and installed 10uf polys when I noticed the amps were no longer reproducing certain frequencies.

They are extremely conservatively rated at 250 watts per channel into 8 ohms and 315 watts into 4 ohms; double that for peak power. In actuality, they should be rated at 350 watts into 8 ohms and around 400 watts into 4 ohms. THD is less than .025% and according to the graph, is less than .01 up to 100 watts. IM is rated at .02% from 250mw to 250 watt. (60 HZ and 7 KHZ mized 4:1)

Weight wise, lets just say they are heavy, well over 50 pounds.

[dynaco_dan]

Hi again Richard;

Try a Leech*.* search, assuming that you used Leech as a descriptor at least once.

This will give you the folder(s) name at least.

[rrcrain]

My chaotic photo storage is relatively organized, falling onto drive D: or E: One drive is a striped 350 gig RAID array, the other is a single 10 gig drive. All photos wind up in one or the other "photo" folders. There are photo files I've scanned from family negatives over 100 years old stored on those drives and I was reasonably organized when I stored them. Finding 6 jpeg files I suspect I forgot to name when they came off the digital camera out of a few thousand may take a while, if I didn't delete them. Given my naming penchant, I'd more likely name the photos "amp" or "circuit board" or "Stereo", all of which failed to uncover what I'm searching for.

Heck, I can find photos of my Labrador when he was 7 weeks old!

How about a photo of the entire system?

I'll be at the Heart of Illinois Fair this weekend manning the Bee Keeping booth for a few hours.

[rrcrain]

I just searched the forum and found the photos. I'd posted what I was able to take a phot of inside the amp without performing a major dissassembly previously. I have to admit, the photos don't show much of the circuit but given how this monster is assembled, I'm a bit more than reluctant to disassemble it for the sole purpose of taking photos.

What can be easily seen in the photos is the size of the power supply transformer and main caps.

Back to my original question, Is there any advantage to replacing the bias cap, a 220uf NP electrolytic with a poly cap? Would the differing ESR alter the circuit somehow that we can't predict? The original bias resistor is carbon. Some say they have a tendency to break down over time; they do if the leads aren't formed properly when they are installed. Is a metal film a better choice or would that create other problems?

I'm "assuming" the 13,000uf caps are good, there's no issues with high volumn distortion. High volumn for me is 60 watts nominal.

[dynaco_dan]

>I just searched the forum and found the photos. I'd posted

>what I was able to take a phot of inside the amp without

>performing a major dissassembly previously. I have to admit,

>the photos don't show much of the circuit but given how this

>monster is assembled, I'm a bit more than reluctant to

>disassemble it for the sole purpose of taking photos.

>

>What can be easily seen in the photos is the size of the power

>supply transformer and main caps.

>

>Back to my original question, Is there any advantage to

>replacing the bias cap, a 220uf NP electrolytic with a poly

>cap? Would the differing ESR alter the circuit somehow that we

>can't predict?

I will need to leave this question to someone more technical.

In my opinion, you have changed the characteristics, by using a poly cap.

The original bias resistor is carbon. Some say

>they have a tendency to break down over time; they do if the

>leads aren't formed properly when they are installed. Is a

>metal film a better choice or would that create other

>problems?

Metal film is a better choice of the two.

>

>I'm "assuming" the 13,000uf caps are good, there's

>no issues with high volumn distortion.

Giving a very thorough looksee with a strong light and small mirror, I would look at the caps for unusual swellings, liquid leaking,any distortion and warts and all, in their shape.

Anything that looks to be deterioration of the caps, I would then replace all of the large power supply caps with Low ESR type, if you can afford them.

The sizes are different for newer caps.

High volumn for me is

>60 watts nominal.

Hi again Richard;

Please don't take anything apart to take a photo.

Just, close enough shots are ok, well lit up, please.

This will be the first time I have ever seen this amp.

If you have read the original article for the Double Barrel amp, you would see that my big bros re-designed his layout to suit his custom made case.

****The following is only if you are capable and can test out the finished product.

Also look at the smaller elect caps within and if possible replace them with 105 degree computer grade, rather than 85 degree standard caps.

If you see carbon composition resistors, perhaps replace them with metal film.

Bypass the power supply caps with a 1.0 uf high voltage rated poly cap.

Because I've not seen this amp before, try to go to Marshall's website and see the various mod's that have taken place before doing anything at all to your amp.

Your amp may or may not follow his Double Barrel/Super Amp precisely.

You should await other, more technical advice, from some of the more technically inclined on this site.

[rrcrain]

>Back to my original question, Is there any advantage to

>replacing the bias cap, a 220uf NP electrolytic with a poly

>cap? Would the differing ESR alter the circuit somehow that we

>can't predict?

>>I will need to leave this question to someone more technical.

>>In my opinion, you have changed the characteristics, by using a poly cap.

That's what I was wondering, and I haven't replaced anything yet except for the input coupling caps.

[rrcrain]

Here's what the circuitry for one channel looks like. Since both are identical, no sense in uploading both.

Bother, Something's happeining to prevent the upload. Says "error on page" when I click on the attachment option.

[dynaco_dan]

Hi Richard;

Check your files on your computer.

Each download can be up to about 4 MB maximum in size PER write-up.

See what you saved your photos as, such as, .JPG, .TIF, etc.

If possible download the largest file size you can for clarity.

Browse your harddrive for that file name, scroll down and choose the .JPG, scroll down and download.

After it tells you it is saved and it's new name, save.

Looking forward to them.

[rrcrain]

The problem was more basic than not finding the file or knowing the file format. I was receiving an error message stating there was an error on the web page. I'll try again tonight and see what happens.

[rrcrain]

Yet another attempt to upload a schematic. ( I do really detest dial-up)

[dynaco_dan]

Hi Richard;

Thank you for the schematic.

I did not realize you were on dialup, sorry.

I switched to highspeed about 2 1/2 years ago and I forget how slow dialup is.

I'll send you an email instead.

[rrcrain]

My wife and I live in a Very rural area where cable is not offered and the dial up is horrible. Verizion has no plans to upgrade the phone lines and the cable companies have no intention of extending into our area. 26.6 is on average, the best we get for dialup

[rrcrain]

email address is "crainrr at gmail dot com"

[dynaco_dan]

Hi Richard;

I already sent you an email a day or two ago via this web site's email system.

I don't have a copy of what I even wrote, sorry.

[rrcrain]

That's what I thought you would do. I just checked the "in box" again and it is empty.

Gremlins perhaps.

Richard

[Pete B]

>Back to my original question, Is there any advantage to

>replacing the bias cap, a 220uf NP electrolytic with a poly

>cap? Would the differing ESR alter the circuit somehow that we

>can't predict? The original bias resistor is carbon. Some say

>they have a tendency to break down over time; they do if the

>leads aren't formed properly when they are installed. Is a

>metal film a better choice or would that create other

>problems?

>

>I'm "assuming" the 13,000uf caps are good, there's

>no issues with high volumn distortion. High volumn for me is

>60 watts nominal.

That cap combined with the input cap determine the low frequency response of the amp, if the -3dB point is 1 Hz, then changing it to 110 uF would, worst case, make the -3dB point 2 Hz. That cap is often made very large to keep the AC voltage across it small for any audio band signal since electrolytics tend to produce more distortion when there is more AC voltage across them. You might be able to get away with a 50 uF poly since they do not have this problem. ESR is not an issue at all since the cap is in series with a 1000 ohm resistor, the ESR is completely swamped by that R. There is little chance that the switch will make a sonic difference, however I'm thinking of making the switch to a poly in my own amps just to avoid electrolytics in the signal path, if I ever get around to it. I will be using low cost polys.

Please note gentlemen that this amp has series connected output transistors because high voltage transistors were not available at the time, they are now and a simpler Super Leach is possible. The Leach is alive and well here:

http://www.diyaudio.com/forums/showthread....4091&highlight=

[dynaco_dan]

Hi Pete:

Thank you for that link.

[rrcrain]

>That cap combined with the input cap determine the low

>frequency response of the amp, if the -3dB point is 1 Hz, then

>changing it to 110 uF would, worst case, make the -3dB point 2

>Hz. That cap is often made very large to keep the AC voltage

>across it small for any audio band signal since electrolytics

>tend to produce more distortion when there is more AC voltage

>across them. You might be able to get away with a 50 uF poly

>since they do not have this problem. ESR is not an issue at

>all since the cap is in series with a 1000 ohm resistor, the

>ESR is completely swamped by that R. There is little chance

>that the switch will make a sonic difference, however I'm

>thinking of making the switch to a poly in my own amps just to

>avoid electrolytics in the signal path, if I ever get around

>to it. I will be using low cost polys.

>

>Please note gentlemen that this amp has series connected

>output transistors because high voltage transistors were not

>available at the time, they are now and a simpler Super Leach

>is possible. The Leach is alive and well here:

Thank you for the feedback. I'll dig out the manual agan and look at the specs on the protection circuit, but I'm very confident it kicks in around 10 hz, give or take a few.

I'd replaced the input caps earlier this year when I noticed low volumn high frequency instruments starting to become non existent. No massive change in the amps sonic performance, but the missing instruments returned and it seemed as if the sound stage expanded.

This is indeed an old amp having been designed and built in the early 80's. One has traveled half way around the world with me and spent enough time on the Atlantic I finally had to rewire it a few years ago, green copper just doesn't conduct all that well. Overall, it's been extremely reliable and while in Colorado, seconded as a small room heater on crisp mornings while I enjoyed the music.

Sound wise, I've only listened to one amp I like better, the Rogue Zeus, but for the money, I'll keep what I have.