Toroid Transformer for Hafler DH-200

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rickrob,

I have two toroids. They are too tall to stack two inside on Hafler chassis. Are you doing dual mono? Will you use two chasses?

If you want a toroid sized to power a DH-220 I have two. Here is a picture of how it was used. The transformer was made by Peachtree Magnetics, Atlanta GA, back in the early 90s.

My price for a toroid is $35 plus postage.
 

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rickrob,

When you convert an amp back to stereo and play it does its transformer still hum?

What is the impedance (load) of your speakers? A bridged amp should never be used with speakers whose impedance is below 8 ohms.

What is the capacitance of the large power supply caps in your amp?

What I am getting at is that the way you are using your amps may cause hum and your present transformers may be OK. Bridging an amp can put it under a different set of load conditions.

Tell us more.

I have no specs on my transformers other than to tell you they were used to power a DH-220 type amplifier used in stereo mode. These were amplifiers made by Smart Devices designed for motion picture theater sound systems that used DH-220 electronics under license. The amps were used in demanding situations and I am sure the toroidal transformers helped out. These transformer are 4.5 inches wide and 3 inches tall. I don't have a scale but I believe they are about the same weight as your Hafler transformers.

Here is a picture of the Smart Design amp from which it came. The heat sinks on either side are exact replacements for those in the DH-220. You can see that the wiring is closer to 18 ga. than 16 ga. The transformer is center tapped and does not have dual secondaries. It has 3 wires on the output side, just like your present Hafler transformer.

Obviously I am not pushing them as a solution. They are good tranformers for a DH-220 and because they are "pulls" they are cheaper than new ones. However, transformers don't really wear out. If you want both, the price is $70 plus $10 for USPS Priority Mail.
 

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I built these in 1983 and then bridged them to mono with the DH-202 bridging board a couple of years later. They drive Dahlquist DQ-10's (8 ohms)

They have always been silent (as best I can remember) , but the other day I noticed the hum from both amps.

I replaced the power supply caps -- the two 10,000uF Sangamo's -- about a year ago. They are still at 10,000uF. I've read that beefing up those caps would help.

I haven't pulled one of the amps out yet and checked it out, but I figure after almost 25 years they may need some additional work. I know they need power switches.

So, I started searching for power switches and toroid transformers to see what I could find. Maybe it's time for a recap on the smaller EL's as well.
 
Dick West said:
...the toroidal transformers helped out. These transformer are 4.5 inches wide and 3 inches tall. I don't have a scale but I believe they are about the same weight as your Hafler transformers.

The DH-220 transformer is 4.6" high and about 5" wide.

There is at least one DH-220 mod-thread which goes live on an intermittent basis -- fwiw, I have replaced the 10,000uF Sangamos with 39,000 Panasonics, bypassed the electrolytics and have not touched the driver board. the DH-220 is in almost constant use here, what a work-horse -- I have it driving a pair of KEF 104ab's in the lab.
 
I agree with you on how robust the DH-220 is. These amps continue to perform well over the years and still command a rather high price at venues such as an eBay auction.

I inserted "DH-220" in my earlier posts because I forgot the original poster states he has a pair of bridged DH-200. Earlier readings on my part found messages in which bridging of the DH-200 was not recommended.

So, question, if the OP's 2 bridged DH-200s have humming transformers, are they humming because the DQ-10 speakers present too low an impedance for his bridged amps?

Also, I have read about humming toroidal transformers. And, have read about the possibility of DC riding on the incoming AC supply as causing humming in transformers. And, we know the DH-200 is rather notorious for having DC offset as original signal transistors drift out of spec, and there is no way to null DC offset on the DH-200. Would DC offset from the amp cause its transformer to hum?

What should the OP do? I have a pair of toroidals sized appropriately for a DH-200/220 amp but will they cure his humming problem? How can he tell in advance of changing transformers to a different pair?
 
In my opinion, if the humming is due to distortion of the AC line voltage, torroidals will hum more (mechanically). The torroidals pictured in the previous replies are not potted. I think potting the Xformers would help.

If there is no DC offset (<50mV) across the load, and the hum is heard with no signal applied to the load, the load being too small for the bridged amp is not the issue.

I'd check for DC offsets, filtering, then winding delamination. If all is okay there, then dirty AC is the likely culprit and unfortunately it's a common problem. ....Potting a torroid my be a good solution.
 
dc riding on the ac line? not likely here in the US, since the lines
are transformer isolated... distortion on the line is possible, but still not likely. loose windings... most likely, especially if the trafo is not potted. most manufacturers until the late 80's-early 90's were dipping their trafos in lacquer, or wax, if anything at all. the primary decision whether to pot or not, is cost. Apt, in their early production amps, had potted trafos, but, because of cost, later went to a trafo dipped in a high temp wax. the new trafo was just as quiet as the potted one, but probably will not stay quiet as long as the potted one. lacquer is fragile, and tends to shake loose, although the newer urethane coatings are a lot more durable than lacquer. try dipping your trafo in urethane.
 
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