Hi! I bought this preamp and in loud volumes I can hear what sounds like a 60Hz hum through my speakers, which are fairly sensitive. Without the preamp in my system, I can hear no hum whatsoever.
What possible investigations and modifications would be advisable to address that 60Hz hum? Thanks in advance!
What possible investigations and modifications would be advisable to address that 60Hz hum? Thanks in advance!
If it is 60Hz as opposed to 120Hz, then you have mains interference from somewhere, could be an Earth Loop?
Check for absence of hum with no inputs and if need be apply short circuits across the selected inputs. Preamp should be dead quiet.
I conducted those tests, but the hum is still there. Just to clarify, I am sending the sound I recorded with my cell phone at 1 cm of the woofer at full volume. I might be wrong, but sounds to me as 60 Hz, even if it is faint.
Am I expecting too much of this preamp? I don't hear anything like it from my solid state receiver or from the class D amp that I am pairing this preamp with. Thanks in advance.
I guess you hear 120 Hz. You can compare with this online generator:
Online Tone Generator - generate pure tones of any frequency
Regards, Gerrit
Online Tone Generator - generate pure tones of any frequency
Regards, Gerrit
I used the visualizations in foobar2000 to get the attached screenshots. It looks like the bulk of it is centered on 60Hz, but there are also upper harmonics that show in the spectrogram.
The pictures don't show, but the 60 Hz peak is about -45 dB in this recording, while the 120 Hz is showing with approximately -60 dB peaks. The recording was made at 1 cm distance from the woofer.
Attachments
Last edited:
I think this amp would really benefit from a few Jupiter caps, a psu cap update (i like JJ...) and Michael Percy has some amazing caddock low wattage resistors under the TF020 line
G'day all, I bought one of these pre's about 2 months ago from ebay with a Quad clone amp 200w for $650. I only wanted the pre (the Quad is of no interest to me, my try it one day) to power a Marantz 8b that is being overhauled by a qualified tube amp whiz.
It has tone controls & defeat button, only wanted it because of the tube rectifiers.
First thing was to change the tubes to USA tubes and remove the window show tube.
Anyway, I have cut the tracks and put jumper wires appropriately for use of the 6X4 rectifiers that I have several of from old radios. RCA, Philips, Radiotron, Miniwatt Mullard, etc... all used and reliable. I used a pair of RCA's.
[I don't use those original tubes, I've seen one put on a fireworks display inside itself (6v6) in one my guitar amps. Funny to watch, I had to tap it to keep it going 'til it died about 2 minutes later. Replaced it with an RCA oldie which is still going strong years later.]
I also replaced the signal caps with Mundorf MCap Supreme.
Replaced out of spec electro. Have ordered new Vishay caps (the 2 big ones) and will probably replace the lot later on.
I will change the metal PIO caps
The 12at's and 12ay's were changed to RCA's triple mica (which I have grown very fond of recently - vintage triple mica). It is costing a bit but that's OK.
I did all that work last Saturday and Sunday and had it running Sunday night.
Not thrilled at the sound but, new caps and tubes are settling. It "is" very accurate and tight.
It is no comparison to my MX110z pre which is running just bonza.
The MC275 just keeps on keeping on. Love it.
So until the Marantz 8b is ready I am using this through the MC275 (w/triple mica 12ax's & 12at's) which run Tannoy SGM10's that I have just replaced the XO's in (Pair VS Clarity ESA speaker TANNOY crossovers) which disappointed me at first. Also done last weekend. Going to install switches so I can compare the XO's - old original vs new.
All this work is settling in and it is sounding a bit better.
The 728 is sounding much better than it did before. I have more that I want to do to it.
Before I commenced on the 728 I took pics of the wiring underneath PCB.
Then I unsoldered the controls PCB and put it aside, then unsoldered all the wires from the main PCB to make it easy to work with.
When that was done I went about soldering posts through PCB so I could run wiring across the top. Easy to remove and refit PCB now.
I'd like thank Tony MA for starting this thread and everyone else for contributing and keeping it going.
Thank you HI Q for the helpful diagrams and information.
I will return later on as I make more changes/upgrades, any problems, breakthroughs, etc...
Cheers & regards,
Andrew.
It has tone controls & defeat button, only wanted it because of the tube rectifiers.
First thing was to change the tubes to USA tubes and remove the window show tube.
Anyway, I have cut the tracks and put jumper wires appropriately for use of the 6X4 rectifiers that I have several of from old radios. RCA, Philips, Radiotron, Miniwatt Mullard, etc... all used and reliable. I used a pair of RCA's.
[I don't use those original tubes, I've seen one put on a fireworks display inside itself (6v6) in one my guitar amps. Funny to watch, I had to tap it to keep it going 'til it died about 2 minutes later. Replaced it with an RCA oldie which is still going strong years later.]
I also replaced the signal caps with Mundorf MCap Supreme.
Replaced out of spec electro. Have ordered new Vishay caps (the 2 big ones) and will probably replace the lot later on.
I will change the metal PIO caps
The 12at's and 12ay's were changed to RCA's triple mica (which I have grown very fond of recently - vintage triple mica). It is costing a bit but that's OK.
I did all that work last Saturday and Sunday and had it running Sunday night.
Not thrilled at the sound but, new caps and tubes are settling. It "is" very accurate and tight.
It is no comparison to my MX110z pre which is running just bonza.
The MC275 just keeps on keeping on. Love it.
So until the Marantz 8b is ready I am using this through the MC275 (w/triple mica 12ax's & 12at's) which run Tannoy SGM10's that I have just replaced the XO's in (Pair VS Clarity ESA speaker TANNOY crossovers) which disappointed me at first. Also done last weekend. Going to install switches so I can compare the XO's - old original vs new.
All this work is settling in and it is sounding a bit better.
The 728 is sounding much better than it did before. I have more that I want to do to it.
Before I commenced on the 728 I took pics of the wiring underneath PCB.
Then I unsoldered the controls PCB and put it aside, then unsoldered all the wires from the main PCB to make it easy to work with.
When that was done I went about soldering posts through PCB so I could run wiring across the top. Easy to remove and refit PCB now.
I'd like thank Tony MA for starting this thread and everyone else for contributing and keeping it going.
Thank you HI Q for the helpful diagrams and information.
I will return later on as I make more changes/upgrades, any problems, breakthroughs, etc...
Cheers & regards,
Andrew.
Some pics I have just taken while music is playing 🙂
All wiring is clear from tubes. I admit I was getting hurried on Sunday night, will tidy up nicely later when I feel like it. Still more to do including output & input wires.
All wiring is clear from tubes. I admit I was getting hurried on Sunday night, will tidy up nicely later when I feel like it. Still more to do including output & input wires.
Attachments
Last edited:
I noticed 2 different values of resistor for the plate of the the 12au7 output in published schematics. Mine had 2.4K but 10K sounded much better. Need to be careful to use a higher power rated resistor. I used 2 watt carbons.
I've been searching for Tube preamps . I'm looking for schematics and details. I'm amazed at how many of these schematics are copied from each other.
