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Need help getting rid of hum

gadget73 said:
I'm actually using Dave's revised values, but with components selected for availability. Dave's specified value for one of those resistors was 2.13M, which is not a thing you can get. I used a 2.0M in series with a 130k to get to the same place.
Same with those two capacitors in parallel. The 1100 in parallel with a 250 makes up the specified 1350pf with parts that can be gotten.

how you get there isn't too critical, just that the final value ends up right.

In that thread should be Mouser part numbers for the specific parts I used. Most of them were selected for small size too so the revised PEC isn't too bulky. I built the thing on perf board because that seemed easier and cleaner.
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Saw the thread. Very interesting. Pretty sure Thorne used those same numbers/values and design.

I might want to try making some. Looking for tiny proto boards. Almost seems like I need to cut them down to get one that isn't bigger than needed. Or could simply cast the circuit in a block of epoxy to make it look clean.
 
Gadget - You're my man ! Tell me how you hooked up the new network leads to the terminals where the PEC leads were connected. Please.
 
Best I can offer is whatever is on the schematic. Pin 1 comes from a 12ax7, the other two go to the selector switch where the original came from. By the schematic, pin 1 on the PEC comes from pin 3 on either V16 or V17. 2 and 3 on the PEC go to the selector switch. Should be right next to one another. PEC pin 2 is for tape head, pin 3 is for RIAA. Some pics of the underside make me think one is mounted right on the selector and one is mounted to a terminal strip near it.
 
Gadg - I know where the PEC pins went before there was the new network. I built a new network. So now I have the PEC with 3 pins and the new network with 2 pins. I'd like to know how to connect both objects given 2 configurations:

1. Both the PEC pins and the new network pins connect to which terminals?
2. The PEC is removed and only the new network's pins are connected to which terminals?

Am I misunderstanding how this works? Thanks, Thorne
 
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If you're keeping the PEC to retain the tape head EQ, you'll need to use PEC pins 1 and 2 and they will go to their original location.
New network goes from pin 1 to where the original pin 3 on the PEC went.

If you're ditching the PEC and want two phono inputs, new network goes from pin 1 to 3, and a jumper from 2 to 3 to get phono in both phono and tape head positions.
At the input jacks, you'll need to add a 100k resistor from the center of each tape head jack to the outer ring in order to get the proper phono cartridge loading.
 
Thanks Gadg...Getting closer. It actually sounds like I had it connected as you say in the PEC + new network configuration. Let me be sure...

Keeping the PEC... Pin 3 of PEC is disconnected leaving PEC pins 1 and 2 connected. The new network has 2 legs, one where the 270k resistor is connected and the other leg.

1. Does the leg with the 270k pin get connected to where PEC pin 1 is connected or if somewhere else, where?

The other leg of the new network...

2. Does it get connected to where PEC pin 3 was connected or if somewhere else, where?

3. Do any of the 3 terminals where the PEC was/is connected have to bridged to other terminals? For example, do terminals 1 and 3 have to be connected?

The way I had it was the 270k leg connected to PEC pin 1 wherever it was connected and the other network leg to where PEC pin 3 was connected. I thought that was correct but hum city was the result.

Then I tried removing the PEC altogether with the 270k network leg connected to where PEC pin 1 was, and the other network leg connected to where PEC pin 2 was connected. I tried that in one channel and I thought I got rid of the hum that way. But when I did it in both channels it sounded worse. Maybe that leg should have been to where PEC pin 3 was connected?

If the network connection to PEC pin 2 should have been to where PEC pin 3 was, then where PEC pin 2 was connected there should be something connected there, maybe a bridge wire from PEC pin 2 terminal to where PEC pin 1 or pin 3 was connected?

I appreciate your help. I've tried hard to make sense of the thread posts and diagrams. It's just that all these questions arise. Thorne
 
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1) yes, both pin 1 of the PEC and the new network go to the same place
2) yes, new network replaces the original pin 3 connection
3) no


If you remove the PEC and do not connect pin 2 to anything, all that happens is the tape head input doesn't work anymore. Having it there or not won't cause hum problems.
If you want 2 phono inputs, a jumper from the original pin 2 to pin 3 location will give you the new RIAA network in both switch positions.


honestly if you've got the PEC still and want to rule out variables, I'd be real tempted to connect it as stock and see what happens. The EQ curve is not grossly off from book spec with the stock network.
 
I like really like the phono section on mine it is pretty nice as is. They mostly got a bad phono section rep from some guy who didn't properly fix his before he ran distortion tests.
 
Thanks gadget. I actually did just 30 minutes ago what you suggested - reconnected the PEC's as stock and removed the new networks. I get hum in both channels with PHONO STEREO selected and nothing connected. Why oh why can't I get the same silence as with AUX. I've read other people who say their PHONO is dead quiet. Where's that train ! :confused:

There was a point last night where I had only the new network connected in one channel and I forget what in the other channel and the hum completely disappeared and I had just that whooshing sound... but from the other channel ! I didn't know what to make of that result. So I just connected the other network the same way on the other channel and it was hum city again in both channels with PHONO STEREO selected and nothing connected.

With the stock PEC configuration, I just now connected a turntable. The hum decreased quite a bit. I played into headphones an orchestral record with quiet parts and the hum decreased even more so. Vinyl surface noise was louder than any hum. Still not as quiet as AUX.

Being somewhat of an audiophile I want the RIAA curve improvement. I know that some say they cannot hear a difference with it, but when it comes to music, I can hear sound differences that most people cannot hear. Dave posted images of the frequency curve differences (with George's help - the last image in this post: https://audiokarma.org/forums/index...er-phono-preamp-and-riaa.600690/#post-7919175). It's true that we're talking about a 1/2 decibel difference in most frequency ranges. I would like to have that 1dB boost in the deep bass. I know...turn up the BASS. :rolleyes:

I thank you for your patience and for answering my 1M questions and now I understand how to connect the network. I'm pretty sure that's how I had it connected so something else is wrong. Would a resistor out of spec cause hum? I'll take it from here. Thorne
 
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tonyg88 said:
They mostly got a bad phono section rep from some guy who didn't properly fix his before he ran distortion tests.
Click to expand...

I think you're thinking of the Dynaco one. The stock Dyna PAS circuit is claimed to be completely out to lunch, though accurate measurements don't agree.

The Fisher one came about from Dave Gillespie's work modeling it. He even said its not far off when stock, the modification is there mostly for those who want it a little closer to perfect.


I've never honestly heard a tube phono stage that didn't have some hum when nothing was connected. If it shuts up enough that surface noise is louder than the hum with headphones on, that might just be as good as its going to get.
 
I just disconnected phono on fisher. Turn volume all the way up and I hear very faint hum the tube swushing sound is louder than hum thou.

I was referring to the stereophile article where I think something similiar happened but was later corrected regarding benchmarking the phono section.
 
Tony - What do you mean "disconnected phono"? You mean you removed the turntable plugs or disconnected the circuit or ? Yeah see, that's my goal and I don't seem able to achieve it. That's how AUX is - a very very faint hum with the volume at max. I get hum with PHONO STEREO at about 10 o'clock and then it gets much louder after 12 o'clock.
 
tonyg88 said:
Time to put the SDS board back in? :)
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Tony - Sorry for the late response. I'm sure you can imagine what I WOULD have said. :yikes:

But thanks for the laugh. While I can't blame the SDS for the hum anymore, I am glad its gone. It's so much cleaner in there now and I can work on something easily if work is needed.

Be happy with yours if it works. Despite what others say, I know it's a reasonable alternative. Thorne
 
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thornev said:
Tony - Sorry for the late response. I'm sure you can imagine what I WOULD have said. :yikes:

But thanks for the laugh. While I can't blame the SDS for the hum anymore, I am glad its gone. It's so much cleaner in there now and I can work on something easily if work is needed.

Be happy with yours if it works. Despite what others say, I'm know it's a reasonable alternative. Thorne
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Exactly
 
Tim - Same hum as before. Definitely 60Hz. Gets louder with volume. Definitely an airy whooshing sound too. And I get some kind of external morse code sound at around 260 Hz. Probably from my network routers that are about 4 feet away.

EDIT: I just noticed that with the stock PEC's installed, with the selector on TAPE HEAD, the hum in the right channel is way louder than the left. Hmmm.

Also I noticed with the selector on PHONO STEREO, when I touch a metal object to pin 3 of the right channel PEC, the hum in the right channel goes away. When I touch metal to the left channel pin 3, the left channel hum gets louder not quieter. Weird behavior.
 
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deleted post - I'm such a schmoo sometimes. I got a ton of hum trying to play a record because I forgot to plug in the extension cables from the turntable. Doy.

Honestly, in PHONO mode my 500-C is the quietest I've heard it in a long time ! :banana:

As a reward to myself I completed the FM stereo de-emphasis mod - replaced R222 and R223 with 16k 1/2w 0.1% metal film resistors. The top end of the FM signal seems stronger. I can cut treble way back. It is a little abrasive on some stations. Happy man. G'night All.
 
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