Tube pre with SS HT- issues (1 Viewer)

[Martin Rendall]

Testing with open plugs doesn't actually prove much. You need o choose an input source, and short the corresponding RCA plugs. Really you should use a resistor, but shorting is better than an open input.

Also, buzzing is different than hum. If the buzzing occurs with power, it is likely (again from the schematic) a problem with the power supply section. Did I mention that I don't like this pre's design? Buzzing with no power... never heard of that. Only heard of 60Hz ground loops, and noisy open inputs. Maybe you should try to contact the manufacturer, and get their take on the problem.

Martin.

 

[Yogi]

"Maybe you should try to contact the manufacturer, and get their take on the problem."

I have and am still awaiting their response.

 

[Scott_N]

Yogi

What is the ouput impedance of the preamp and the input impedance of the your amp? Most people say that the input impedance of the amp should be 10x the ouput impedance of the preamp.

 

[Yogi]

Guys, I found out that the hum with the preamp shut off was due to the way the wires from the power switch to the trannie were routed. Once I moved them the preamp was dead quiet when turned off. But with the preamp turned on there is still the same level hum from both channels. Also this hum changes its magnitude when I switch sources, so I am assuming its again a routing problem. I am going to take some copper tape and shield all the wires and ground them to the chassis of the pre and see how that works.

Thanks again guys for all the help and suggestions.

Regards,

 

[MannyE]

I just went through this thread and I don;t think anyone mentioned the tubes themselves...I think that if you are still getting some kind of audible noise, especially on one side only... but it seems you have fixed that...

Well, at any rate, did you try switching the tubes and seeing if the "signature" of the niose moves with the tube?

I built a Bottlehead Foreplay (in stock form it has got to be the best value in tube premps at only 149 bucks) and was dismayed by the loud buzzing present even after 24 hours of warmup. It turned out to be the tubes themselves. I replaced them with a pair of NOS RCAs (12AU7) that cost me 30 bucks and everything went silent!

So maybe you should take a look at the tubes to get you the rest of the way?

I have to make one comment though...someone mentioned that the Foreplay was 450 dollars and while I can see how one could get up there by getting the Anticipation boards and the Black Gate caps and the silver wire and even go beyond that when you start talking about upgraded trannys and other stuff. None of this stuff is needed!

If I had heard the 450 price point, I would have NEVER dabbled in tubes, and much less DIY tubes... but at 149, I thought it was worth a shot... NOW..:

http://home.bellsouth.net/p/s/commun...upid=23441&ck=

OK...the cabinet cost me 50 bucks to get veneered, but you have to admit, they look like 5 grand. What's more important, is they sound great. The amps cost a little more (I think the whole amp project was about 750 or so with all the extra doodads).

 

[Martin Rendall]

MannyE,

That was me. I mixed up US$ and CDN$ in that case. Add Anticipation $75, NOS Mullards, $30, budget stereo attenuator $30, upgraded output caps $50 = around $335 USD.

BTW: your cabinets look incredible!

Martin.

 

[MannyE]

Well, yeah, in fact I think there are some folks who if you add up the Cardas connectors, tantalum resistors, silver wire, fancy aluminum cabinets, megabuck attenuators, Solen Caps (and the aforementioned BLack Gates), copper shielding, and the list can go on forever, have way more than 1K in the Foreplay...of course it does not even resemble the original kit anymore and is probably worth more than the sum of its parts compared to retail offerings, but the point I wanted to get accross was that it still sounds phenomenal at under 200 dollars (US). That being the cost of the basic kit and some NOS tubes (unless you get lucky and Doc sends you a nice pair of RCAs or Mullards)

Thanks for the compement on the cabinets! I went to a cabinet maker in my area and convinced the guy to sell me some remnant ebony veneer cheap (like free almost) and install it for 50 bucks right over the stock alder bases from bottlehead. Had I known they were so simple, I would have made the bases myself...next time maybe. Next time I will also remember to have the chassis anodized a nice gold or champagne tone!