Help finding hum on tube phonostage

Hi all, I wondering if you can help me find a 60hz hum on my tube phonostage? My brother suggested to start with twisting pair the heater wires?

Also, it has very poor bass. Is this because of wrong cartridge loading? Can I change this? Or can change the riaa curve somehow?

The unit is a Space Tech Lab. I don't have the schematic, but it looks pretty simple. (See attached)



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kevinkr

Administrator
Paid Member
What sort of test equipment do you have at your disposal?

Did you build this yourself from provided parts or purchased it as shown?

There are so many issues I can see with this build I am not even sure where to start.

Short the phono inputs to ground and measure the hum at the output as a starting point.

The power transformers and other PSU components are pretty close to the very low level circuitry found in a phono pre-amplifier.

As to the poor bass response it is not unlikely given the overall quality of this build that the EQ is incorrect.

Sorry.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Kevin: believe it or not, this is actually a commercial product someone is selling for money.

Space-Tech Laboratory - High-End Audio 

Of course, besides the fine construction and design evidenced here, they also sell remote-controllable Schumann resonators and this "device". It's depressing to think that there's people out there who are that gullible.
 
This was one of their first builds I think. But still, no excuses. I paid $400 used, a few years ago. It sounds very transparent, and "promising". I have basic soldering skills, and a voltmeter, I don't mind learning or buying some testing equipment if you tell me what to get. it will be a good starting project, and look forward to correcting it.
 
Looks like two sockets.

Honestly I would consider this a learning opportunity and start fresh, consider it part of the cost of learning. I'm thinking unfortunately that there is not much worth salvaging here.

A good starting point would be the Salas phono and his 6V6 line stage. They could be combined on a single chassis. Lots of insight on the supporting threads.

I would strongly recommend building the power supply on an entirely separate chassis. Getting the hum out will be much simpler!

I've shared a number of designs here as has Stuart, my perception is that my designs at least are well beyond newbie friendly.
 
Kevin: believe it or not, this is actually a commercial product someone is selling for money.

Space-Tech Laboratory - High-End Audio*

Of course, besides the fine construction and design evidenced here, they also sell remote-controllable Schumann resonators and this "device". It's depressing to think that there's people out there who are that gullible.

I'm completely flabbergasted and don't know quite what to say other than perhaps OMG with regard to that device.. That's pretty blatant :spin:
 
I understand that you want to re-use the chassis, it could save some drilling and filing etc.

I would keep it simple. If you want to reuse the 12AX7 then search up the RCA Phono and look for information on improved component values for it that were posted by Thorsten, somebody should be able to point you to it.

Perhaps it would be possible to build the power supply inside a separate screened section at one end of the chassis. Maybe even buying a small metal box that you could fit inside the original chassis to give it some isolation. A solid state rectifier into a filter capacitor avoids a power supply choke which can be a source of ugly magnetic field induced hum.

Do make sure you are aware of safety requirements when working with mains powered power supplis and high voltage tube supplies.


P.s. What are those i.c's ? Is this an opamp phono with 'tube sound' added in ?
 
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Hearinspace

Member
Paid Member
2008-06-03 5:18 am
The guy from Space Tech may be out there on the edge (I was in his shop once, over 10 years ago, clearly a prolific builder,) but he has been in business for a long enough time to have a following and I wonder how that could be the case if he isn't capable of making something that doesn't hum.

As for the build quality, yeah, I get it, but I have a Fisher FM tuner that sounds great even though it looks like they mixed a bunch of parts in a bowl of molten solder and threw the resulting batter real fast so the solder would just be getting cool enough to freeze everything in place as it hit the inside of the chassis. Randomization of strays, I think they call it.:D

If it were mine, before tearing it apart I would follow Sy's first advice and trace out the circuit (into a drawing) and check all the connections.
 
This is an absolutely disgusting standard of construction for a commercial product. It is hard to see but I cannot find a connection from the earth prong of the mains IEC connector to the chassis. This is a basic safety violation and could lead to severe electric shock. This manufacturer should be reported to the safety and trading standards authorities

Cheers

Ian
 
I'm no expert but, tracing wiring as much as I can in first photo, I'd say -
PCB is MC boost pre-amplifier, you can see shielded cables going to the front panel MC/MM selector switch.
I'd suggest the 2 transformers are to avoid a single custom job.
First transformer has 110v primaries in parallel for 110v, twisted yellow mains side wiring.
Second transformer has twisted blue main side wiring, possibly 12v secondary feeding MC pre-amp. Can't for the life of me see where HT is taken from. Possibly the 2 elco's adjacent to second transformer & hidden under all the wiring are part of HT supply. All this is guesswork - check for yourself before electrocuting yourself.

The spiders web of wiring and unsupported components frightens me. I wouldn't even lash up a test circuit like that (my limited experience has taught me that if 2 wires can touch, they will).
If you are not using MC I'd be inclined to ditch that part. That'll simplify the layout and give you more room to play with. And if you are using MC, you probably need a better phono amp.