Hey All,
I'm having a problem with a monoblock I'm building. I'm using Pete Millet's uniamp front end which I've used before with great results. But I have a problem with this one's input. It is an unbalanced input, i.e. an RCA jack, and I'm not sure how to connect it. The transformer is an EDCOR WSM 10k/10k 1:1. I've attached a snipet of the original schematic. Its a differential amp using two stages of 6SN7's. My question is what is the best way to connect the input? I've tried using the RCA + on one end of the primary and - on the other which I think is wrong, very noisy. And with the primary center tap grounded or not, still noisy. I'm thinking of the RCA + at one end of the primary with the other end tap grounded, i.e. SE and the RCA negative to circuit ground. But if that is done wouldn't I need a load resistor between the input and the transformer?
I'm having a problem with a monoblock I'm building. I'm using Pete Millet's uniamp front end which I've used before with great results. But I have a problem with this one's input. It is an unbalanced input, i.e. an RCA jack, and I'm not sure how to connect it. The transformer is an EDCOR WSM 10k/10k 1:1. I've attached a snipet of the original schematic. Its a differential amp using two stages of 6SN7's. My question is what is the best way to connect the input? I've tried using the RCA + on one end of the primary and - on the other which I think is wrong, very noisy. And with the primary center tap grounded or not, still noisy. I'm thinking of the RCA + at one end of the primary with the other end tap grounded, i.e. SE and the RCA negative to circuit ground. But if that is done wouldn't I need a load resistor between the input and the transformer?
Attachments
I have a pair of those same transformers. It sounds like you're using the transformer to split phase, correct?
The way I used my Edcor 1:1's was mimicking what's there for the Sowter transformer. PRI 1 is incoming signal, PRI 3 to the shield of the input connector. The input connector should not ground to the chassis (it should be left floating, shield will be grounded at the source). The CT on the primary is unused.
Then the push-pull signal comes from the secondary SEC 4 (+) and SEC 6 (-). I have a 12k load resistor going from SEC 4 to SEC 6. 4 and 6 go to the input tubes' grids. You can ground the CT on the secondary (5) to the signal ground to chassis *inside* the amp, but don't connect the PRI 3 "ground" to the chassis ground.
Anyway, that's what worked out for me, using it as a phase splitter/input xfmr to a similar medium-mu triode pair.
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The way I used my Edcor 1:1's was mimicking what's there for the Sowter transformer. PRI 1 is incoming signal, PRI 3 to the shield of the input connector. The input connector should not ground to the chassis (it should be left floating, shield will be grounded at the source). The CT on the primary is unused.
Then the push-pull signal comes from the secondary SEC 4 (+) and SEC 6 (-). I have a 12k load resistor going from SEC 4 to SEC 6. 4 and 6 go to the input tubes' grids. You can ground the CT on the secondary (5) to the signal ground to chassis *inside* the amp, but don't connect the PRI 3 "ground" to the chassis ground.
Anyway, that's what worked out for me, using it as a phase splitter/input xfmr to a similar medium-mu triode pair.
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I usually connect the + input to one end of the primary, and then the ground of the RCA to the other end and leave the whole winding floating. The earth connection is made at the source. In this setup the whole primary is left floating in the amp. I have never had issues doing it this way and its usually very quiet.
The reason I do it this way is it also allows me to connect an XLR in parallel without compromising the balanced connection. Its not necessarily the correct engineering approach but it works.
Loading the secondary is usually a very good idea since otherwise they tend to ring badly. I often go as low as 1K across the whole secondary but I am usually stepping down through the transformer which presents a higher load to the source.
Shoog
The reason I do it this way is it also allows me to connect an XLR in parallel without compromising the balanced connection. Its not necessarily the correct engineering approach but it works.
Loading the secondary is usually a very good idea since otherwise they tend to ring badly. I often go as low as 1K across the whole secondary but I am usually stepping down through the transformer which presents a higher load to the source.
Shoog
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I followed your advice to see what would happen. With the amp's input shorted it's quiet. I tried it with two different preamps and then it buzzes badly. Any ideas?
Pin 2 or pin 3(Lundahl) should connect to the chassis, not to the ground. If the hum persists, then check all ground lines with a DMM. You should have a ground star scheme.
Does the amplifier hums by its shelf, i.e. no connection?
Then try a 100nF cap between RCA ground and chassis.
Humming, if it's bad, usually shows a ground loop. If it's still there, check the filament PSU. You should ground it via a small cap, or a wire.
That should be enough to troubleshoot hum.
Does the amplifier hums by its shelf, i.e. no connection?
Then try a 100nF cap between RCA ground and chassis.
Humming, if it's bad, usually shows a ground loop. If it's still there, check the filament PSU. You should ground it via a small cap, or a wire.
That should be enough to troubleshoot hum.
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