Two units in one chassis
« on: September 20, 2017, 09:34:03 AM »
What are the pitfalls of housing a couple of separate amps/units with two separate power supplies in one chassis?  I've been considering adding a tube section in my project that is transistor based.  The output of one would go to the next as if it was two separate units but under the same roof.

Is it asking for trouble?  Pros and cons?

Adam


andyfromdenver

Re: Two units in one chassis
« Reply #1 on: September 20, 2017, 11:14:14 AM »
the pitfalls are as you can imagine (PT flux inducing hum in inductors or ins/outs transformers).  A very careful layout will reduce the hum to a pro audio level.  Perhaps there is a custom PT that can supply all the voltages required. 
The LA610 (among so many other designs) employes a single PT. B+, heaters, phantom pwr, single supply or bipolar (forgot, it's been a while since I mapped it out) for an opamp.
I would strive for a single PT, however with careful orientation and distancing sensitive stages from noisy stages etc. multiple pwr supplies can be used.

My LA2A has a separate PT for two LED meter circuits, I was careful to test for noise interference and rotate/position accordingly and it's crazy quiet.

Shielded ins/outs, and possibly using a toroidal PT are a good idea.

I'm not "against" DC heaters, cause that would be silly, but I am a firm believer that AC heaters are 100% adequate. But what the heck, if you have the room, go DC heaters :-)


Andy
« Last Edit: September 20, 2017, 11:17:27 AM by andyfromdenver »
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JohnRoberts

Re: Two units in one chassis
« Reply #2 on: September 20, 2017, 11:31:54 AM »
There is a small benefit from a common chassis ground. You need good signal discipline passing signals around between subsections (use differentials).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Two units in one chassis
« Reply #3 on: September 20, 2017, 03:32:09 PM »
Thanks guys.  If the main transistor based section has output transformers before the tube section, will it work to have balanced XLR output pre-tube AND float the - part of the signal at the tube input?

Maybe it'd be better to have the tube input come from XLR inputs anyway?

Adam

Re: Two units in one chassis
« Reply #4 on: September 20, 2017, 03:38:06 PM »
There is a small benefit from a common chassis ground. You need good signal discipline passing signals around between subsections (use differentials).

JR

Hi John,

I was curious about that and wondered if sharing a chassis ground actually could/should yield a benefit, and I'm glad that thought was on the right track.

I guess one benefit of keeping it as a separate unit is mobility... oh so many things to consider.

A
« Last Edit: September 20, 2017, 03:42:44 PM by 80hinhiding »

Re: Two units in one chassis
« Reply #5 on: September 20, 2017, 03:42:13 PM »

I'm not "against" DC heaters, cause that would be silly, but I am a firm believer that AC heaters are 100% adequate. But what the heck, if you have the room, go DC heaters :-)


Andy

My power transformers are both from Hammond.   Models 266J48 and 269JX.

The low voltage higher current winding on the 269JX doesn't have a center tap so maybe I should go with DC?  I remember Ian telling me at one point if it had a CT it could be connected to virtual ground from the CT of the higher voltage winding.  I think I've worded that right.

Thanks for your help Andy, hope you're doing well.

Adam

andyfromdenver

Re: Two units in one chassis
« Reply #6 on: September 21, 2017, 01:13:53 PM »
Hi Adam. can you better elaborate on what systems you want to run into what?
maybe a rough diagram or schematic?  I believe in a post you touched on a junction with an impedance match to consider, as well.
transistor thing with OT -> tube thing input?
In regards to your heater question. It will be a matter of cost and space. fwbr heater circuit with large mfd filtering stage or regulator, or dropping resistors and filtering.
or leave AC and create an artificial centertap, or "humdinger" (a low ohm 1-2W pot with the wiper to ground).
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

abbey road d enfer

Re: Two units in one chassis
« Reply #7 on: September 21, 2017, 03:01:40 PM »
the pitfalls are as you can imagine (PT flux inducing hum in inductors ). 
Sometimes having two PT's is better, if you can arrange them in humbucking.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

andyfromdenver

Re: Two units in one chassis
« Reply #8 on: September 21, 2017, 03:52:52 PM »
Sometimes having two PT's is better, if you can arrange them in humbucking.
oh wow!  Thank you, I didn't know/ consider that.
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

Re: Two units in one chassis
« Reply #9 on: September 23, 2017, 08:14:48 AM »
I have a transistor based line output amp (virtual earth mix amp) driving a CMOB-2L output transformer (1:1 600 ohm), which goes to an XLR output connector.  That's pretty much the end of the line for that system.

Here's what I'm considering.  The signal could next go to Ian's Twin Line Amp, setup as an unbalanced passive summing bus amp where the audio goes to IN1 and IN2.  The schematic for this is on his website.  The thing is, I don't want to unbalance what is going out of the master outputs of my XLR that is connected to CMOB-2L prior to the tube section, and I'm a little confused about how to avoid doing that.

I'd like to maintain a balanced output at one stage, and continue on unbalanced.  My thought is that I'd have to pass the audio signal to the tube stage prior to the CMOB-2L, while it's still unbalanced.. and send it to the Twin Line Amp setup in a virtual earth summing bus mode.

If anyone can offer some clarity on this I'd be grateful.  I'm running out of space in my chassis so I may build it separate anyway, but I need to get my head around this concept for sure.

Like, what happens to one copy of an audio signal when you just leave it hanging in mid air?

Adam


Re: Two units in one chassis
« Reply #10 on: September 23, 2017, 08:32:49 AM »
In regards to your heater question. It will be a matter of cost and space. fwbr heater circuit with large mfd filtering stage or regulator, or dropping resistors and filtering.
or leave AC and create an artificial centertap, or "humdinger" (a low ohm 1-2W pot with the wiper to ground).

From the primary side of the 269JX power transformer, grey wire to the white wire, I get 7.4VAC from the secondary low voltage winding.  With the primary side connected Black to White it's closer to 7VAC.  I think I should use Black to White on the primary, since the secondary is then closer to the 6.3V that the heater needs.  Is 0.7V, +- a small margin, going to drop under load without another resistor in series?

Adam

andyfromdenver

Re: Two units in one chassis
« Reply #11 on: September 23, 2017, 04:19:08 PM »
From the primary side of the 269JX power transformer, grey wire to the white wire, I get 7.4VAC from the secondary low voltage winding.  With the primary side connected Black to White it's closer to 7VAC.  I think I should use Black to White on the primary, since the secondary is then closer to the 6.3V that the heater needs.  Is 0.7V, +- a small margin, going to drop under load without another resistor in series?

Adam
When I get a chance, I will look at Ian's scheme, and comment on that.
 I love the 125V primary option on Hammond's PTs. Under load, both will come down, but I  use the 125V option (the black wire) exclusively for U.S. wall outlet levels.  If you are only very lightly loading it (say one tube for example) even the 125V option might be uncomfortably over, possibly facilitating the choice for DC heaters. 
tbc in this reply! lol.
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

Re: Two units in one chassis
« Reply #12 on: September 24, 2017, 07:37:43 AM »
When I get a chance, I will look at Ian's scheme, and comment on that.
 I love the 125V primary option on Hammond's PTs. Under load, both will come down, but I  use the 125V option (the black wire) exclusively for U.S. wall outlet levels.  If you are only very lightly loading it (say one tube for example) even the 125V option might be uncomfortably over, possibly facilitating the choice for DC heaters. 
tbc in this reply! lol.

Ian's design uses a 12AX7WA and two 6922 tubes.  I may also use a 6x4 rectifier, so heater currents will total 1.5A I think.

Since my filament winding doesn't have a center tap (doh I forgot to get that option), I am thinking about creating an artificial center tap from two low value resistors and then connecting the mid point to an elevated voltage.  I read this can be sufficient, but in some cases it may be beneficial to use DC on the input tube(s).

Hopefully on the right track here.  I need to finish the main part of my project first though (still lots of work left on that), this is kind of preparation work for the next stage, getting my head around it.

I've gained so much respect for professional mixers throughout the course of my DIY mixer project over the past year, especially in recent months as I've been able to dedicate a LOT more time to my project and realized the scope of what's involved.  I am still a fan of "beefy" looking and things that do inject a little distortion however, I now equally admire the other side of the coin and all the creative, excellent work that is put into making circuits that just work really well without any hassle and the design that goes into everything... right down to component count and chassis, space, layout and the list goes on.

I've been tracking lately and using a Zed R16 mixer and it's certainly not hindering the music.  In fact, when I get to work it's a reliable tool.  I apologize for calling it boring at one point last year.  It's not.

I'd like to thank everyone for helping me so much.  I'm so close to finishing my own mixer, but the last 20% is taking the longest.  Hope to have pictures and sound samples in the coming months.

Cheers

Adam