Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 8Len8 on March 21, 2015, 09:24:59 am
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I've get a Phaez amp that has 2 individual inputs, one for each channel. I decided instead of using an AB box that I'd install a switching circuit to route a single input to either channel.
Originally the amp had each channel's preamp tube connected to its own input jack.
I got a Granger optoFET channel select board:
http://grangeramp.com/shop/index.php?main_page=product_info&cPath=37&products_id=493 (http://grangeramp.com/shop/index.php?main_page=product_info&cPath=37&products_id=493)
I powered it off a wall-wart and hooked it up like this (all connections via coax):
(http://i1329.photobucket.com/albums/w541/anzelde/schemeit-project-2_zpseybov3bm.png) (http://s1329.photobucket.com/user/anzelde/media/schemeit-project-2_zpseybov3bm.png.html)
My only problem is that I hear a slight pop when switching from the clean to dirty channel. I've read that FET based channel select circuits should have quiet switching. Is there a pull-down resistor or something that I'm missing that could be causing the pops? Even after I added the RC high pass filters the pop was still there...
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Give a look to the way Sluckey do the commutation on his latest amp
no optocoupler, a simple DPDT switch
http://home.comcast.net/~seluckey/amps/dual_lite/Dual%20Lite.pdf (http://home.comcast.net/~seluckey/amps/dual_lite/Dual%20Lite.pdf)
K
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My mistake. Here's the correct schematic of how the circuit is wired:
(http://i1329.photobucket.com/albums/w541/anzelde/schemeit-project_zpstqyxzoqo.png) (http://s1329.photobucket.com/user/anzelde/media/schemeit-project_zpstqyxzoqo.png.html)
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I would like to try a pair of 1M resistors across the switch (and maybe remove the 470k resistors ?)
(http://i.imgur.com/feWST8n.jpg)
K
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I'll try that, but don't the tube inputs need a DC path to ground?
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I wrote
(and maybe remove the 470k resistors ?)
the question mark is because the presence of the in series 0.1 cap confuses me
K
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I added the 0.1 uF caps (the the 470K resistors) as a suggestion from another forum where I posted this problem. Before then I just had the Granger Opto module directly connected to each preamp tube input pin.
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I think if your search the forum for "channel switching" or "channel pop" you will find something about the 1M K is referring to. FWIW here's a schematic with relays but the I think the 100k's and big caps are for switching type spikes, pops.
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Try with the 1M resistors and without the cap and the 470k resistors
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If the previous didn't solve, try also this configuration
(http://i.imgur.com/lJd6YOp.jpg)
(http://i.imgur.com/6mhMEvv.jpg)
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I really would like to know if using a simple DPDT switch the pop is present
K
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Try with the 1M resistors and without the cap and the 470k resistors
If the previous didn't solve, try also this configuration
I really would like to know if using a simple DPDT switch the pop is present
K
I'll try the 1M resistors next week when I can get my hands on some. I'd need a pretty quiet DPDT switch to try out your recommendation so I wouldn't be confusing the switch noise with the amp circuit noise :).
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Try with the 1M resistors and without the cap and the 470k resistors
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If the previous didn't solve, try also this configuration
(http://i.imgur.com/lJd6YOp.jpg)
(http://i.imgur.com/6mhMEvv.jpg)
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I really would like to know if using a simple DPDT switch the pop is present
K
I tried the top mod (1M resistors) and the pop is still there. I'm not sure removing the caps would do anything.
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I don't say it can solve, but try to remove the cap
a charged cap when switched on give a pop
K
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Those dirty channels generally have huge gain making the input very sensitive. I always move the switching down the line someplace where it isn't near as sensitive. I used to use H11F3's, which is an infrared LED controlled bilateral FET. The things could handle a 30Vp signal, so it was possible to use it after the first stage. I don't know what your device will handle, but I'd bet that it is similar.
By the way, I quit using H11F3's because they added a subtle sound to the amp even when only used to short the signal to ground. In fact, I have found that any SS device touching your main tube preamp circuit in any way adds a sound to it.