noise in right channel after changing sweet whispers

[Ritchie]

I recently removed the sweet whispers volume controls and installed a stereo 100k dact attenuator in my ext. foreplay iii.
I retained the original 33k resistors and wired the attenuator the same way as the whispers.
Upon trying the new dact I had alot of hum on the right channel which is where the dact was installed.
I removed the a.c. feeding the power switch and this got rid of most of the hum but it is still noisier than the left channel and it was not there with the sweet whispers. The wiring from the dact is not shielded but the lengths are fairly short.Is this part of the pre very prone to noise depending on wiring? The noise does not vary with volume level and is the same even at minimum volume setting.
Any ideas?

Thanks,
Ritchie

 

[mchurch]

Ritchie;

I just finished changing to a 20K DACT stereo pot. I am still experimenting with the padding resistors so I have a different one in each position of the selector as I have a DACT 5 position 4 pole to add but i want to get the padding right before I do any installing of the new switch. This will give me two extra inputs. There is absolutely no hum even when fully turned up. I did mount it on the opposite side to the A/C switch so I don't know if that helped or not. I did have some very minor hum before I changed to the new attenuator but while changing it I went over all the solder joints and re-heated some of them. I did not twist the signal wires either. The fact that you are getting hum with minimum volume setting would suggest something amiss with the soldering. Also did you check the connections to the DACT to ensure Ground, Input, and Output are connected the same way as the SW. Remember the input and output of the DACT have to be reversed. I don't think this would cause the hum. I would stick with checking all the unit to see if something got shifted during the install or a ground wire got broken.

Cheers

Mike

 

[Ritchie]

All wiring is ok and it is wired with the input and output reversed as the original were. Connections are also ok.
The noise I am getting is very minor not like an open ground or something major.
I have to get within 2 feet of my speaker to hear it so it's not really a big deal when listening, but it is on the right channel only which does bother me enough to want to track it down. I also switched tubes and it wasn,t that.
I also think my amps are to sensitive with the foreplay as it gets loud quickly with volume increments. It's a little better with the dact but still very sensitive. My amplifier is a homemade eiclone which is very close to an eico hf-87.The input tube is a 12ax7 and the input sensitivity is 0.38v for full output (speakers=90db). The original eico came with a 500k input pot, I think I know why now.
Paul's article on amp and speaker sensitivity is bang on. I don't mind modding my amp to lower the input sensitivity.
Pots on the input might be the easiest but changing the front end would also be an option, I'd have to seek Paul's advice if changing the front end though.
I think alot of the old tube amp designers had it right when they designed with volume pots on the input of amplifiers.

Thanks,
Ritchie

 

[Paul Joppa]

...I think a lot of the old tube amp designers had it right when they designed with volume pots on the input of amplifiers. ...

Yeah, I think so too! It's what you call an "engineering compromise" - the best solution would be to have every component follow a strict standard of sensitivity and signal level, but since that's never going to happen you look for the next best thing - the ability in this case to adjust sensitivity as needed.

The high sensitivity does play a part in this puzzle. I suspect the high impedance (100K vs. 15K for the Whispers) also plays a part, making  the wiring more susceptible to capacitive coupling of hum. If that is the case, shielding or moving the wires or reducing the impedance are the logical answers. I know of no way to tell which is the best of them, short of trying each of them and taking data.

We have gone back and forth with the power switch - it's always going to be quieter if it's back by the IEC entry point and far from the small-signal area, but it's darn inconvenient to reach around the (hot!) tubes to get at it.

 

[Ritchie]

Thanks, I'll try some of those suggestions and hopefully resolve it.
The power switch was not a problem on my Foreplay until I installed the dact att. which turned out to be more sensitive to ac pick-up.

Ritchie

 

[Ritchie]

I believe the main source of the noise was just to high a gain into my amps and it was magnifying any little noise including alot of tube rush.
I ended up wiring the dact attenuators in normal configuration and I put a voltage divider ahead of the attenuator input.
Seems fine now and I can actually run my pot between 8:00 and 2:00. Before I couldn't get past 10:00.
Anyone having this type of issue really needs to read Paul's article titled "signals,noise and signal to noise ratio".
It really sheds light on the difficulty of mismatched components and gain issues. Not sure what effect the voltage divider is having on the sound but it's staying in as the noise floor is much better and I can get much deeper into the pot.

Thanks,
Ritchie