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Old 22nd June 2019, 03:09 PM   #31
Mark Johnson is offline Mark Johnson  United States
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Post #1 of this thread says

Quote:
Originally Posted by MagnumOpus View Post
I really like the Qualtek q300 power intet module so that I will Get from digikey. Partsconnexion has the transformer and the Chassis wil most likely be a Galaxxy. The Hammond 6L6 use is to tight inside. I need space to work
which makes me think that the Mouser Shared Shopping Cart may not include the AC power inlet module, or the power transformer, or the chassis. It might be worth further investigation.

edit- I also don't see any connector(s) for the left and right input signals. RCA jacks and/or 3mm TRS jack seem not to be on the list.

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Last edited by Mark Johnson; 22nd June 2019 at 03:24 PM.
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Old 22nd June 2019, 06:11 PM   #32
adydula is offline adydula  United States
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Hmm...

Just looked again and the BOM doesnt seem to have the transformer?

Alex
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Old 22nd June 2019, 09:33 PM   #33
dB Cooper is offline dB Cooper  United States
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Personally (not being a 'cable guy') I could live without an IEC power cord, and it seems like just using a regular cord with a strain relief and soldering to the board would alleviate crowding in that corner with that particular case...

What does the optocoupler do in this circuit?

I don't recall seeing a power line fuse in the schematic but I might have missed it... Shouldn't there be one?
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Old 22nd June 2019, 10:39 PM   #34
adydula is offline adydula  United States
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The 4N35 optocoupler does a few things -

The optocoupler has two sides when looking at the schematic, the diode and the transistor. They are linked optically, not electrically, so the two sides of the optocoupler can share different voltages that don't effect the other side. as the current change in the LED side of the opto it will glow brighter or dimmer, which controls how much the transistor side conducts - in this circuit the current through the LED is directly equal to the mosfet current, and as it gets brighter it controls the BJT, whereby the BJT "burns up" the excess dc bias voltage.

The optocoupler appears to be a variable resistor in parallel with the inside 10k resistors - it changes the gate bias with the collector-emitter junction as the opto coupler looks at the current through the mosfet sources. The LED part of the opto has a 1.2v constant drop, this is used in conjunction with R18 to set bias current across the source resistors. If the current is too high it will make the LED brighter, that modulates the base of the transistor, and the collector-emitter junction will decrease its apparent resistance in parallel with the inside 10k resistors, changing the ratio of rail voltage to ground, decreasing the amount of voltage on the gates, and keeping the bias stable as the load swings.

Is a simple solution - its a single part and it automatically adjusts. If there is any drift the optocoupler will compensate immediately No resistors to measure across and potentiometers to adjust


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Alex
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Old 23rd June 2019, 06:45 PM   #35
dB Cooper is offline dB Cooper  United States
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Interesting...
Seems like this may not be ideal for IEM use with the 'stock' gain. I suppose the gain could be set appropriately but I'd still like to be able to change that easily.... which means a switch. I know enough to follow Dynaco etc style instructions but don't trust myself to follow a schematic or do the gain resistor calculations correctly. This is one of the things I like about my agar-modded O2, I have the two gain settings 'dialed in' for 22Ω IEMs and 250Ω fullsizers respectively.
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Old 24th June 2019, 08:11 PM   #36
adydula is offline adydula  United States
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I dont have any IEMs here to try. I did try 32 ohm MSR7's to the 600 ohm T1's and in all cases the amp seemed to work very well with the volume well away from the low end for tracking issues etc..

Maybe someone with IEMS will comment
Alex
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Old 24th June 2019, 11:37 PM   #37
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by dB Cooper View Post
Interesting...
Seems like this may not be ideal for IEM use with the 'stock' gain. I suppose the gain could be set appropriately but I'd still like to be able to change that easily.... which means a switch. I know enough to follow Dynaco etc style instructions but don't trust myself to follow a schematic or do the gain resistor calculations correctly. This is one of the things I like about my agar-modded O2, I have the two gain settings 'dialed in' for 22Ω IEMs and 250Ω fullsizers respectively.
Gain is set by R1 and R4 for right and R12 and R8 for left channel by relationship:

G_right = (R1+R4)/R1

G_left = (R12 + R8)/R12

Currently R1&R12=1k and R4&RR8=4.75k so Gain is 5.75x or 15.2dB. This is a perfectly fine value for most moderate sensitivity headphones.

If you have 115dB/mW bal armature IEMs, a gain closer to 6dB may be better. Change R1 and R12 to 4.7k and you should have about 6.1dB gain.

Note that this approach unbalances the opamp inputs a bit but preserves the feedback value. Then the input 1k on the other side (R2 and R7) can also be changed to 4.7k to keep things balanced. Or you could change R4 and R8 to 1k and that also gets 6dB gain, but changes the feedback value and might affect stability. it will probably be fine and nothing is going to explode so don't worry too much.

Alternatively, an attenuator (resistor divider) can be placed on the input after the input jack.

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Last edited by xrk971; 24th June 2019 at 11:43 PM.
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Old 28th June 2019, 09:18 PM   #38
martin clark is offline martin clark  Europe
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Alternative observation:

R4, R8 are driven by the power output - so driving a low impedance is not a problem, and therefore and alternative to get down to 6dB gain as XRK's good suggestion above, you could just reduce R4, R8 to 1K, and all will be well in terms of fairly-well balanced input impedances at the opamp.

C2,C7 increased to 470pF would exactly compensate.


NB - XRK's previous suggestion or this, are both 'quieter' than just attenuating the input, to amplify it again; and that just might matter if you want to drive IEMs.

HTH.

Last edited by martin clark; 28th June 2019 at 09:21 PM.
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