Distorted hiss
I'm having a spot of bother with my preamp. I'm getting a low volume intermittent distorted hiss from the right channel.
Most of the time the thing sounds perfect but every now and then I get said noise. I can't hear it when the music is on as it very quiet.
When I was constructing I managed to get the valve socket for the 6sn7 out by one place. I can't remember if it was clockwise or anticlockwise. I had powered up and nothing popped but obviously the thing didn't work. I resoldered the socket and retested and it worked.
I had this noise first appear after a couple of months of use. The volume control didn't affect the volume but did that scratchy pot thing when you don't use a blocking cap.
I changed the tubes as I bought a spare set when I thought I had fried them and the noise disappeared for a bit.
However it has come back. It's very quiet but this time the volume control does affect the volume of the noise but it's not doing the scratchy pot thing.
Is it likely I damaged some of the resistors when I powered up with the incorrect alignment of the tube. (I think I dodged a bullet, that could have been nasty!) Or maybe the power aupply.
Could it be the tubes get too hot through inadequate ventilation. The chassis is big but not much space around it.
Or could it be mains Bourne interference from either plug in network adapters, switch mode psus etc. I have been investigating things like the fridge switching on and off but I don't seem to be able to stimulate the noise.
Any ideas to the cause or likely culprits to investigate.
Thanks.
I'm having a spot of bother with my preamp. I'm getting a low volume intermittent distorted hiss from the right channel.
Most of the time the thing sounds perfect but every now and then I get said noise. I can't hear it when the music is on as it very quiet.
When I was constructing I managed to get the valve socket for the 6sn7 out by one place. I can't remember if it was clockwise or anticlockwise. I had powered up and nothing popped but obviously the thing didn't work. I resoldered the socket and retested and it worked.
I had this noise first appear after a couple of months of use. The volume control didn't affect the volume but did that scratchy pot thing when you don't use a blocking cap.
I changed the tubes as I bought a spare set when I thought I had fried them and the noise disappeared for a bit.
However it has come back. It's very quiet but this time the volume control does affect the volume of the noise but it's not doing the scratchy pot thing.
Is it likely I damaged some of the resistors when I powered up with the incorrect alignment of the tube. (I think I dodged a bullet, that could have been nasty!) Or maybe the power aupply.
Could it be the tubes get too hot through inadequate ventilation. The chassis is big but not much space around it.
Or could it be mains Bourne interference from either plug in network adapters, switch mode psus etc. I have been investigating things like the fridge switching on and off but I don't seem to be able to stimulate the noise.
Any ideas to the cause or likely culprits to investigate.
Thanks.
I dont think its getting very hot. The top of the amp is barely warm and the shelf above it again is just slightly warm. Its also only from one channel.
I think ive narrowed it down to 2 possibilties based on the heater coil taking diverting the power where it shouldnt go. Either rotations not good...
1. Its connected the heater supply to pin 1 at r3 (dont know which side of the board is which)
2. its connected the heater supply to pin 6 at the leds.
I think the second one is likely to have done more damage although im surprised they didn't pop.
I just hope its not damaged the transformer but i would think that would be a constant failure.
Looks like im going to have to drag it out and get the scope on it.
I think ive narrowed it down to 2 possibilties based on the heater coil taking diverting the power where it shouldnt go. Either rotations not good...
1. Its connected the heater supply to pin 1 at r3 (dont know which side of the board is which)
2. its connected the heater supply to pin 6 at the leds.
I think the second one is likely to have done more damage although im surprised they didn't pop.
I just hope its not damaged the transformer but i would think that would be a constant failure.
Looks like im going to have to drag it out and get the scope on it.
If you have, or can borrow an infrared thermometer look at the temperature of the D-MOSFETs. They shouldn't be running more than ~65⁰ C if the 6SN7 is running at 8mA.
FWIW, I have found that the NOS Tung-Sol 6SN7's are relatively quiet compared to the EH.
also, the Impasse THD+N% is comparable to one of the $25k line amplifiers reviewed in Stereophile.
FWIW, I have found that the NOS Tung-Sol 6SN7's are relatively quiet compared to the EH.
also, the Impasse THD+N% is comparable to one of the $25k line amplifiers reviewed in Stereophile.
Thanks jack, yes, ill check the temp later today. I have a thermometer.
I am using a tung sol, not sure how old they are. The one i took out has 15 05 printed on the base so week 15, 2005? if thats in fact what its refering to.
I did have a golden lion 6922 but swapped that out for a EH when i tried changing the tubes.
The other channel is doing it a bit but MUCH quieter, i have to put my ear up close to the mid and i can 'just' hear it and more prominent from the tweeter but no other running noise or the like.
Ok, good to know it should be practically silent and Im not chasing my tail as it were.
Do D mosfets heat up fast?
I am using a tung sol, not sure how old they are. The one i took out has 15 05 printed on the base so week 15, 2005? if thats in fact what its refering to.
I did have a golden lion 6922 but swapped that out for a EH when i tried changing the tubes.
The other channel is doing it a bit but MUCH quieter, i have to put my ear up close to the mid and i can 'just' hear it and more prominent from the tweeter but no other running noise or the like.
Ok, good to know it should be practically silent and Im not chasing my tail as it were.
Do D mosfets heat up fast?
Ok, Its on the bench, I plugged in the interconnect so it shorted hot to ground.
Ive got my scope on the cold outputs. Ive plugged in to a duff amp so at least theres something referencing to ground. I cant pick up anything unique to the RH channel with my scope.
Ive been monitoring the d mosfets.
Q1 and Q3 hovers around 38
Q2 about 48 and low and behold
Q4 (on the right hand channel) is about 67 with the lid off so its obviously too hot..
Bear in mind that my thermometer is cheap so could be 10% error either way.
Ive checked out R4 and R10, the 3.75w 5% wirewounds.
R10 measures in circuit when coldish 12k00 is dropping 95v when powered.
R4 is measures in circuit when coldish 12k44 dropping 98v when powered.
R19/R5 (in parallel with adjust) 262 and 275
r6 and r7 118 to 120
If Q4 is causing noise because its too hot or damaged, would there be cross talk to the other side?
Regards. D.
Ive got my scope on the cold outputs. Ive plugged in to a duff amp so at least theres something referencing to ground. I cant pick up anything unique to the RH channel with my scope.
Ive been monitoring the d mosfets.
Q1 and Q3 hovers around 38
Q2 about 48 and low and behold
Q4 (on the right hand channel) is about 67 with the lid off so its obviously too hot..
Bear in mind that my thermometer is cheap so could be 10% error either way.
Ive checked out R4 and R10, the 3.75w 5% wirewounds.
R10 measures in circuit when coldish 12k00 is dropping 95v when powered.
R4 is measures in circuit when coldish 12k44 dropping 98v when powered.
R19/R5 (in parallel with adjust) 262 and 275
r6 and r7 118 to 120
If Q4 is causing noise because its too hot or damaged, would there be cross talk to the other side?
Regards. D.
I'm not sure what I'm meant to do with this. The plate voltages both measure the same.
Is q4 running much hotter because it's damaged or because its having to dissipate more due to the smaller voltage drop across the 12k5 which measured only 12k?
Although it does make the noise even before it's got up to temperature.
Is q4 running much hotter because it's damaged or because its having to dissipate more due to the smaller voltage drop across the 12k5 which measured only 12k?
Although it does make the noise even before it's got up to temperature.
Hi sy. Yes tried putting a scope on the output but I'm struggling to track the noise on a scope. It's not brilliant, it's an owon and tbf it's a bit noisey itself. I did swap both tubes out as my initial thought was i had damaged them. I initially thought that had cured it but the noise soon came back. I'm worried that I'm going to start eating through tubes.
OK. I'll try that. The LEDs are lit but as I have wondered if they got damaged when I miss connected the 6sn7 when I built it. They would have had the heater supply connected to the top of them via the heater coil.
It does sound a bit like that damp carbon comp plate resistor staticy rustly hiss you get from old guitar amps.
When it's not doing its noise thing (which is very quiet) the thing is silent. Thanks
It does sound a bit like that damp carbon comp plate resistor staticy rustly hiss you get from old guitar amps.
When it's not doing its noise thing (which is very quiet) the thing is silent. Thanks
I run out the D_mosfets for the 6SN7 with an 18V suply, "+" probe attached to the tab of the first device through a milliAmmeter. The negative supply to the bottom of the set resistor. With the values SY first specifies you probably have 7.3 to 7.6mA. I have a 10 turn 10k potentiometer i put in parallel with R_Set until it just reaches 8.0mA.
Measuring the value of the pot gives me the correct parallel resistor value -- or you might get lucky and the combo of the two resistors equals one standardized value.
Measuring the value of the pot gives me the correct parallel resistor value -- or you might get lucky and the combo of the two resistors equals one standardized value.
Thanks guys,
I would think for 25k it would be somewhat better matched than that. In reality though is the peak at 100khz a problem?
I take it you run the milliameter in line rather than measuring the voltage across the 12.5k (like I the artical) because it does away with having to know exactly what the the 12k5 is and gives you the direct value you what.
Similarly, does sy's artical measure the voltage drop as it's a bit simpler in practical terms?
I would think for 25k it would be somewhat better matched than that. In reality though is the peak at 100khz a problem?
I take it you run the milliameter in line rather than measuring the voltage across the 12.5k (like I the artical) because it does away with having to know exactly what the the 12k5 is and gives you the direct value you what.
Similarly, does sy's artical measure the voltage drop as it's a bit simpler in practical terms?
OK. Yes, I remember when modding an alpha 5 CD player I ended up with an opamp that was unstable and oscilated very high (can't remember, but well out of audio) but it didn't sound very good. So I guess the problem is whats causing it to amplify 100khz a little more rather than the fact that 100khz is being amplified more.
Regarding the d MOSFETs, you (sy) mentioned that you had a lot of varience with them and discarded 2/3. Should I be matching those first? I recently had to match vbe for a load of transistors for a ladder filter. Is there a similar method for d MOSFETs and which parameter?
Thanks
Regarding the d MOSFETs, you (sy) mentioned that you had a lot of varience with them and discarded 2/3. Should I be matching those first? I recently had to match vbe for a load of transistors for a ladder filter. Is there a similar method for d MOSFETs and which parameter?
Thanks