Power Transformer Surprise
« on: January 16, 2018, 06:57:49 PM »
I've been testing a NE5532 as a line receiver, feeding two BA283 output stages.. on a breadboard.  The power transformer was a 266J48 from Hammond.. running it in parallel mode, which can put out 24VAC 2A on the secondary.

I swapped out the 266J48 for a 187E24 by Hammond (24VCT 2.4A), and to my surprise.. the circuits sound different.  I swear.

I thought it might be a fault of the original 266J48 which I had accidentally overheated when I first started out over a year ago, so I had bought another of the same model as a replacement.  To rule that out I tried the replacement 266J48.  They are all different, but the two 266J48s are closer than the 187E24.

I preferred the 266J48.  To describe what difference I heard is most prominently in the mid to upper frequency range.. the 187E24 changed that quite a bit.  Guitars a little crunchier, snares and hihat transients a bit annoying to me.  Overall not as smooth of a performance from the audio circuits sound wise.

I'm shocked by this in a way I guess.  The manufacturer says it shouldn't make a difference.. I heard otherwise.  And I didn't want to hear otherwise.

I checked and noticed the 187E24 puts out a higher secondary voltage so I added a little more resistance on the + rail in a second test, hoping that was all it was.. but no, even at the right voltage for the circuit it's still a different tone.

What could be going on?  I was going to try toroidal, and opted for the cheaper model thinking there wouldn't be much of a difference if any.. except maybe in noise.

Adam



Re: Power Transformer Surprise
« Reply #1 on: January 17, 2018, 08:36:40 AM »
Has anyone else here experienced this?  Do you just try  different Power Transformers out to see what you like best for the application?

I plan to use two power transformers, one for a dual rail regulated supply for ICs, another for a single rail supply.  I'll probably get up to 1A being drawn on the single rail when the mixer is all done, so I figured 2A or 2.4A will be sufficient spec wise.

Adam

Newmarket

Re: Power Transformer Surprise
« Reply #2 on: January 17, 2018, 09:42:42 AM »
Has anyone else here experienced this?  Do you just try  different Power Transformers out to see what you like best for the application?

I plan to use two power transformers, one for a dual rail regulated supply for ICs, another for a single rail supply.  I'll probably get up to 1A being drawn on the single rail when the mixer is all done, so I figured 2A or 2.4A will be sufficient spec wise.

Adam

Do you have a drawing of your power supply circuit from the mains input to the op amp rails ?

Re: Power Transformer Surprise
« Reply #3 on: January 17, 2018, 09:54:46 AM »
Do you have a drawing of your power supply circuit from the mains input to the op amp rails ?

For the tests I've been using one power transformer at a time to supply the IC and the discrete circuits.  From the secondary VAC it goes to a full wave bridge rectifier, made from 1N4007 diodes.  10400uF filtering, then goes to a 120ohm power resistor to bring down the voltage since the overall load is so small right now.  I split the main rail with two 30K 1% resistors and added a couple caps.  Referenced the virtual ground for the IC.

I can draw that up later if it would help.

I realize I'll likely be better off with a regulated supply especially for the ICs, but this is what I've been trying for now.

Adam

Newmarket

Re: Power Transformer Surprise
« Reply #4 on: January 17, 2018, 10:27:15 AM »
For the tests I've been using one power transformer at a time to supply the IC and the discrete circuits.  From the secondary VAC it goes to a full wave bridge rectifier, made from 1N4007 diodes.  10400uF filtering, then goes to a 120ohm power resistor to bring down the voltage since the overall load is so small right now.  I split the main rail with two 30K 1% resistors and added a couple caps.  Referenced the virtual ground for the IC.

I can draw that up later if it would help.

I realize I'll likely be better off with a regulated supply especially for the ICs, but this is what I've been trying for now.

Adam

Do you have the instrumentation to measure the actual voltages and noise / ripple accurately (at least in terms of relative values) ?

Re: Power Transformer Surprise
« Reply #5 on: January 17, 2018, 11:26:39 AM »
Do you have the instrumentation to measure the actual voltages and noise / ripple accurately (at least in terms of relative values) ?

Unfortunately no I don't, not yet.  All I have is a digital multi meter.

ruairioflaherty

Re: Power Transformer Surprise
« Reply #6 on: January 17, 2018, 01:37:25 PM »
As someone who makes his living from trusting his ears I advise you to stop trusting your ears :-)  You need to step up your measurement game, comparing stuff like this by ear only is a recipe for crazy. 

I'm sure you already have an audio interface and REW is free and will give you powerful FFT and distortion testing capability.  Something as simple as oscillation in one opamp will cause the exact symptoms you are hearing. 

REW, a $100 used analog scope and a halfway decent multimeter will get you very far.






Re: Power Transformer Surprise
« Reply #7 on: January 17, 2018, 04:37:09 PM »
As someone who makes his living from trusting his ears I advise you to stop trusting your ears :-)  You need to step up your measurement game, comparing stuff like this by ear only is a recipe for crazy. 

I'm sure you already have an audio interface and REW is free and will give you powerful FFT and distortion testing capability.  Something as simple as oscillation in one opamp will cause the exact symptoms you are hearing. 

REW, a $100 used analog scope and a halfway decent multimeter will get you very far.

Thanks. I agree it can make you a bit crazy.

I always like taking breaks and doing a recording session, and then coming back to it.  I do wish I had the measuring equipment already.

A