Domg,
Can you point me to an accurate and exact schematic of your amplifier, Please?
One of us might be able to see where you can ground some grids of various stages (without damaging anything), in order to see:
either how early, or how late, in the amplifier signal chain that the hum is occurring.
That is one troubleshooting technique, and it might be a good start to find the cause of the hum.
Can you point me to an accurate and exact schematic of your amplifier, Please?
One of us might be able to see where you can ground some grids of various stages (without damaging anything), in order to see:
either how early, or how late, in the amplifier signal chain that the hum is occurring.
That is one troubleshooting technique, and it might be a good start to find the cause of the hum.
Hi @6A3sUMMER,
Thanks for looking. I’m building monoblock EL34s using MkII Baby Huey boards - schematics attached for amp and power supply boards.
Much appreciated,
Dom
Thanks for looking. I’m building monoblock EL34s using MkII Baby Huey boards - schematics attached for amp and power supply boards.
Much appreciated,
Dom
Attachments
Domg,
Was the amplifier working without hum, before you installed the Sowter 3575 Input Transformer?
Please draw a schematic that includes the XLR input connector, the Sowter 3575 primary, 3575 secondary, and the circuits of the input triodes grids.
Are you using negative feedback to the one input grid?
Depending on all those details, it may or may not, be possible to ground the two input grids.
Is it possible that the XLR, 3575, or input grids are not wired correctly?
Remember, the two input triodes grid's circuits and resistors are wired different if you are using a single ended input and negative feedback; but are wired exactly the same if you are using a balanced input.
Using an input transformer: balanced to balanced input transformer; or a balanced to unbalanced input transformer, or an unbalanced input to balanced input transformer; will change how the input stage and negative feedback circuitry (if negative feedback is used).
If the input circuit wiring is all OK, than a cathode to filament leakage on one triode, and no cathode to filament leakage on the other triode would cause an un-balanced hum from one triode, but not the other triode . . . that is not common mode hum, it is differential hum (not likely, but possible).
I use JJ 12AX7 from eurotubes.com, they are extensively re-tested when they get them from the JJ factory. By the way, the JJ 12AX7 tubes use the superior spiral wound filaments, not the more usual and common up-down, up-down, up-down filaments of many other 12AX7 tubes.
With No global negative feedback connected, then before powering the amplifier, you could:
Connect a 1uF 50V or more poly or other plastic cap from pin 5 of one EL34 to ground (cap from grid to ground); and use another 1uF cap for the other EL34 pin 5 to ground.
If you get still get hum, it is likely caused by mis-wiring or bad/wrong part(s) in the the output circuit.
When you power down the amplifier, do not forget to take the 1uF caps out.
What is the signal source that you are using and connecting to the XLR input?
Is that signal source balanced, or is it single ended?
Was the amplifier working without hum, before you installed the Sowter 3575 Input Transformer?
Please draw a schematic that includes the XLR input connector, the Sowter 3575 primary, 3575 secondary, and the circuits of the input triodes grids.
Are you using negative feedback to the one input grid?
Depending on all those details, it may or may not, be possible to ground the two input grids.
Is it possible that the XLR, 3575, or input grids are not wired correctly?
Remember, the two input triodes grid's circuits and resistors are wired different if you are using a single ended input and negative feedback; but are wired exactly the same if you are using a balanced input.
Using an input transformer: balanced to balanced input transformer; or a balanced to unbalanced input transformer, or an unbalanced input to balanced input transformer; will change how the input stage and negative feedback circuitry (if negative feedback is used).
If the input circuit wiring is all OK, than a cathode to filament leakage on one triode, and no cathode to filament leakage on the other triode would cause an un-balanced hum from one triode, but not the other triode . . . that is not common mode hum, it is differential hum (not likely, but possible).
I use JJ 12AX7 from eurotubes.com, they are extensively re-tested when they get them from the JJ factory. By the way, the JJ 12AX7 tubes use the superior spiral wound filaments, not the more usual and common up-down, up-down, up-down filaments of many other 12AX7 tubes.
With No global negative feedback connected, then before powering the amplifier, you could:
Connect a 1uF 50V or more poly or other plastic cap from pin 5 of one EL34 to ground (cap from grid to ground); and use another 1uF cap for the other EL34 pin 5 to ground.
If you get still get hum, it is likely caused by mis-wiring or bad/wrong part(s) in the the output circuit.
When you power down the amplifier, do not forget to take the 1uF caps out.
What is the signal source that you are using and connecting to the XLR input?
Is that signal source balanced, or is it single ended?
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Hi @6A3sUMMER
All sounds good with the Sowter 3575 disconnected and a jury rig single-ended input (both with and without negative feedback), so looks like the input transformer connection is the problem.
Sketch of the balanced input wiring attached. I’d still prefer to be able to use balanced input if possible, from a balanced Mytek DAC/preamp.
Dom
All sounds good with the Sowter 3575 disconnected and a jury rig single-ended input (both with and without negative feedback), so looks like the input transformer connection is the problem.
Sketch of the balanced input wiring attached. I’d still prefer to be able to use balanced input if possible, from a balanced Mytek DAC/preamp.
Dom
Attachments
Domg,
Might be the order of connections, or might not . . .
What does Sowter's data sheet describe as the application of the 3575Z?
Input differential / balanced, across pins 1 and 3?
Un-balanced with pin 3 grounded, and pin 1 single ended 'hot' signal?
Able to be applied both of the above ways?
You are using the secondary in a differential / balanced way (center tap grounded).
But you did not draw the 3575 to the amplifier's input circuitry connections for . . .
With global negative feedback.
Without global negative feedback.
How far is the 3575 from the power transformer, and from the B+ choke?.
What is the angular orientation of the 3575 coils, versus the power transformer coils, and the choke coils?
Are you using a Magnetic Steel Chassis?
Are your DAC and Preamp rated to have the DCR of the 3575 primary across their output pins 2 and 3?
Might be the order of connections, or might not . . .
What does Sowter's data sheet describe as the application of the 3575Z?
Input differential / balanced, across pins 1 and 3?
Un-balanced with pin 3 grounded, and pin 1 single ended 'hot' signal?
Able to be applied both of the above ways?
You are using the secondary in a differential / balanced way (center tap grounded).
But you did not draw the 3575 to the amplifier's input circuitry connections for . . .
With global negative feedback.
Without global negative feedback.
How far is the 3575 from the power transformer, and from the B+ choke?.
What is the angular orientation of the 3575 coils, versus the power transformer coils, and the choke coils?
Are you using a Magnetic Steel Chassis?
Are your DAC and Preamp rated to have the DCR of the 3575 primary across their output pins 2 and 3?
Hi @6A3sUMMER
1. From the Sowter datasheet for 3575 (link here)
2. For ‘with global negative feedback’ the 0 and 8Ohm OPT taps were connected the X3-1 and X3-2 inputs shown on the schematic.
3. See attached photo for location and orientation of the 3575 input transformer (more photos in post #1,459).
Dom
1. From the Sowter datasheet for 3575 (link here)
“APPLICATION
For reduction of most kinds of noise caused when equipments are connected. Ideal as an input transformer for a valve or transistor amplifier to provide a balanced, isolated line input which will then accept balanced or unbalanced sources of any impedance. Unbalanced to balanced conversion and vice versa. For use where it is important not to load the driving circuit. For helping to ensure Electromagnetic Compatibility (EMC) compliance. High level capability means it will not be overloaded in most applications.”2. For ‘with global negative feedback’ the 0 and 8Ohm OPT taps were connected the X3-1 and X3-2 inputs shown on the schematic.
3. See attached photo for location and orientation of the 3575 input transformer (more photos in post #1,459).
Dom
Attachments
Please see the attached file.
click and paste from my text inserts excess empty lines between each line of text. So I converted the text to a .pdf file.
I hate software!
Marketing gobble de gook descriptions are not the same as hard specifications of Sowter 3575, and the same goes for the signal sources output characteristics.
And, without more description about your signal source's output impedance, DC offset voltage, and capability to drive the Sowter 3575's primary's unknown DCR and unknown impedance, we do not know if your signal source is oscillating or distorting as a result of the 3575 loading it.
Is there a B+ choke.
Where is it?
I can not find it in the photographs.
If present, where is the choke in relation to the Sowter 3575?
click and paste from my text inserts excess empty lines between each line of text. So I converted the text to a .pdf file.
I hate software!
Marketing gobble de gook descriptions are not the same as hard specifications of Sowter 3575, and the same goes for the signal sources output characteristics.
And, without more description about your signal source's output impedance, DC offset voltage, and capability to drive the Sowter 3575's primary's unknown DCR and unknown impedance, we do not know if your signal source is oscillating or distorting as a result of the 3575 loading it.
Is there a B+ choke.
Where is it?
I can not find it in the photographs.
If present, where is the choke in relation to the Sowter 3575?
Last edited:
Mods made as per @6A3sUMMER attachment and Baby Hueys now both singing beautifully. This completes my PlyFi adventures. Many thanks to those who generously shared advice.
Dom
P.S. in answer to 6A3’s last questions - the output impedance of the Mytek DAC is 75 Ohms, but DC offset voltage not given in specs. See attached photo for locations of Sowter 3575 input transformer and choke.
Dom
P.S. in answer to 6A3’s last questions - the output impedance of the Mytek DAC is 75 Ohms, but DC offset voltage not given in specs. See attached photo for locations of Sowter 3575 input transformer and choke.
Attachments
Thanks Francois.
@OldHector - Agreed, a protective bottom plate is on my list of finishing touches (along with rubber feet, a final coat of lacquer and possibly some fancy caps). Just looking around for something that will still allow some ventilation.
The speakers are a Troels Gravesen design, The Loudspeaker III http://www.troelsgravesen.dk/The-Loudspeaker-III.htm
@Domg "Just looking around for something that will still allow some ventilation"
look at the hardware store for perforated metal sheet
https://www.diy.com/search?term=perforated+sheet
look at the hardware store for perforated metal sheet
https://www.diy.com/search?term=perforated+sheet
I think you will struggle if you are looking in the US, which sounded like Aliexpress is out of the question. You can buy cheaper toroid covers if you just want to disguise the toroids and don't need the chrome covers and potting.
How many transformers is that? 6? or chokes too? Have you dropped Toroidy a line to see if there is a way to save a few bucks, e.g. a discount for the size of the order?
How many transformers is that? 6? or chokes too? Have you dropped Toroidy a line to see if there is a way to save a few bucks, e.g. a discount for the size of the order?
if you know a company that chrome-plates inexpensively, that might be a cheap alternative!?
https://aliexpress.com/item/1005005080334914.html
https://aliexpress.com/item/1005005080334914.html
@bloozestringer,
Consider the naked toroids from Toroidy. Same transformer sans prettiness.
https://www.tme.com/ux/en-us/details/ttg-el34pp/toroidal-transformers/toroidy/
If you buy from www.TME.com the prices are very reasonable, shipping is quick and (last time I bought a few months ago) the cost per transformer was $9.99 shipped from Poland to my address in USA. Note that the $9.99 was for a package under 5 kg IIRC, so two transformers bought together with packaging material exceeds that limit.
Consider the naked toroids from Toroidy. Same transformer sans prettiness.
https://www.tme.com/ux/en-us/details/ttg-el34pp/toroidal-transformers/toroidy/
If you buy from www.TME.com the prices are very reasonable, shipping is quick and (last time I bought a few months ago) the cost per transformer was $9.99 shipped from Poland to my address in USA. Note that the $9.99 was for a package under 5 kg IIRC, so two transformers bought together with packaging material exceeds that limit.
@Francois G
Thanks. I was just looking at the supremes on the Toroidy site. I could get some naked ones and add covers for asthetics. Appreciate your help as always.
Also, I’ve sent a PM to @Algar_emi to see if he had any more PS boards. I have a set of his boards and also a set of the ones from Prasi, so I could technically use the Prasi one with bowling’s mods if I have to. I’m looking for one more to covert to monoblocks. So if anyone has an extra PS pcb they might want to part with 😉
Thanks. I was just looking at the supremes on the Toroidy site. I could get some naked ones and add covers for asthetics. Appreciate your help as always.
Also, I’ve sent a PM to @Algar_emi to see if he had any more PS boards. I have a set of his boards and also a set of the ones from Prasi, so I could technically use the Prasi one with bowling’s mods if I have to. I’m looking for one more to covert to monoblocks. So if anyone has an extra PS pcb they might want to part with 😉
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