Fine Attenuator

[amirhosein]

I just joined the Bottlehead family I had a few hiccups as I forgot to solder some of the joints but now everything is fixed. However, there is a problem that I cannot figure out. The fine attenuator doesn't seem to change the outout volume at all. The only thing that was a bit off is the voltage on the side A kreg which reads 16.4v as opposed to sude B reading 11.1v. Please let me know if I should post any pictures of any specific parts

[Paul Birkeland]

Posting pictures of the area around the fine attenuator could be very helpful.

[amirhosein]

Here is a picture of the fine attenuator just in case.

[Paul Birkeland]

The joints on the switches look good, but the joints on the terminal strips look like they could use a little extra solder.

Does the coarse attenuator work as expected?

There are a few different reasons that the fine attenuator wouldn't be doing its job, but I don't see any evidence of those conditions, and some that I thought of would've caused issues with one channel of the coarse attenuator as well.

[amirhosein]

The joints on the switches look good, but the joints on the terminal strips look like they could use a little extra solder.

Does the coarse attenuator work as expected?

There are a few different reasons that the fine attenuator wouldn't be doing its job, but I don't see any evidence of those conditions, and some that I thought of would've caused issues with one channel of the coarse attenuator as well.

Thanks I'll double check and report back. Somehow the capacitor in the middle of socket C had fallen off too, I soldered it back in. One more thing is terminals 19 and 22 ( the ones with the white wires connecting them to OA on the C4S boards show 220 k ohms, not aure if that's expected or not. The coarse attenuator seems to work fine by the way. On terminal 1 thebresistamce goes feom 20-21.6-23.7-26.7-31.44-40 and on terminal 5 it goes from 40-40-40-26.75-31.47-40 from lowest gain to highest. One weird thing is sometimes I don't have sound in the right channel until I turn the coarse up to 0 db and then it gets sound and stays no matter what gain I set it to afterwards, like it needs a kickstart or something. What is that problem that could affect the coarse one as well?

The resoldering of the terminals didn't seem to change anything.

P.S. this might be because I'm using IEMs right now but there is also a high pitch moise going on all the time which might ormight not be related.

Terminal 2 to 30 has a resistance of 4.227 mega ohms jf that matters.

P.S. 2 I resoldered all the connections and soecxially terminals that seemed like they might be problematic. The hiss and the problem with the coarse attenuator went away but the fine attenuator is still not working. Terminal 5 is also reading the same resistances as terminal 1 right now 20-21.6-23.7-26.731.5-40.

Fix: Ok I just realized what the problem was Silly mistake. The lower lug on the fine attenuator wasn't connected to terminal 7 on the coarse was, it was connected to the lower lug on the coarse attenuator. Working now! Thanks.

Final problem: There is white noise in the left channel that lowers in volume as the amp warms up, this noise is present even without any input connected but the right channel ia dead silent. Is this a grounding issue? And the right channel still goes out sometime when changing between inouts or high and low impedance and seems to come back on if I restart the amp. (Kreg on side a is still 16.5V) Swapping the  tubes seemed to change this but after rebiasing the tubes twice it went back to just a bit of noise in the left channel and also an almost imperceptible noise in the right one.

[amirhosein]

Is 16.5 v Kreg on side A anything to worry about comoared to 11 v on side B?

[Paul Birkeland]

No, that's not particularly concerning.  Where you will see issues is when Kreg goes below 3V or above 30V.  Below 3V and the regulator won't work properly.  Above 30V and the regulator will blow.

[amirhosein]

No, that's not particularly concerning.  Where you will see issues is when Kreg goes below 3V or above 30V.  Below 3V and the regulator won't work properly.  Above 30V and the regulator will blow.

Awesome, Thank you ao much Payl I huess it would be ridiculous to expect a cpfire audio IEM to have no hum on a tube amp I'll invest in an attenuator for it.

[Paul Birkeland]

If you are getting some tube rush (I doubt you're hearing hum), you can add a dedicated 1/8" mini jack for IEMs, or you can buy/make an attenuator for them. For the Mainline, I would use a 32 ohm and 3 ohm resistor in series to pad down each driver, which would give you roughly -20dB of noise and likely plenty of output.  You could also use a 32 ohm resistor in series with a 6 ohm resistor, then have another 6 ohm resistor that could be switched in.  This would give a little extra flexibility.

If you would like any of these drawn up, let me know.  From what I can remember, a lot of the attenuators available are 16 ohms, which isn't ideal, and they are pretty expensive for something you can very easily make yourself (you've built a Mainline after all, it's time to put those skills to work!).

[amirhosein]

If you are getting some rube rush (I doubt you're hearing hum), you can add a dedicated 1/8" mini jack for IEMs, or you can buy/make an attenuator for them. For the Mainline, I would use a 32 ohm and 3 ohm resistor in series to pad down each driver, which would give you roughly -20dB of noise and likely plenty of output.  You could also use a 32 ohm resistor in series with a 6 ohm resistor, then have another 6 ohm resistor that could be switched in.  This would give a little extra flexibility.

If you would like any of these drawn up, let me know.  From what I can remember, a lot of the attenuators available are 16 ohms, which isn't ideal, and they are pretty expensive for something you can very easily make yourself (you've built a Mainline after all, it's time to put those skills to work!).

To be honest I have mo idea which one would be better Would the 1/8" jqvk replace the 1/4" or should I cut a new hole? Also, I suppose this would work with the resuator route balance connection as well?

[Paul Birkeland]

You could drill an extra hole or replace the 1/4" jack. 

If you need to work on both the TRS and the balanced jack, I would just make an adapter that plugs into the Mainline.

[amirhosein]

I found a nice tutorial for the adapter. https://diyaudioheaven.wordpress.com/headphones/headphone-attenuation-adapter/ but I would imagine that I could just add yhe resistors before the  1/4" connector to have the same effect, but permanently  just for that plug right?

[Paul Birkeland]

I wouldn't just add those to the 1/4" jack.  That will leave the amp loaded down if you go to use it with other headphones (I didn't think about this before).  It is really best done with a plug in adapter just like that article describes. 

[amirhosein]

I finished the convertor! It was diddicult to make all the reaiators fit inaide the XLR connector without breaking amy of the connections but I did it in the end