An inexpensive audio transformer with DIY projects

[boomertech]

I experimented with the T-B pedal today and have a few more recommended suggestions. First I will share my observations with the circuit that we finalized on Jan 16.

The 9V supply that I used has an output of 9.34Vdc.

Original circuit (Jan 16):

U1b output amplifier clipped long before the U2 primary drivers clipped. There is just too much gain at U1b. Solution= reduce output gain for U1b. (Suggested revision on Mar 16)

U2 transformer primary drivers clipped on the negative portion of the signal at -4.2V with the OPA2134. The supply pins of U2 (pins 8 & 4) measured 9.08V and -7.61V. Solution= to increase headroom, an opamp with more output horsepower is needed.

Another thing that I noticed was the input signal at pin 3 on U1a was slightly shifting bias levels. The R1 2M2 bias resistor is too light for the BJT front end of the 4558 opamp. To keep the approx 1M ohm input impedance, this opamp will need to be changed to a FET or CMOS front end.

Changes that I made today:

-Installed the Mar 16 suggested revisions for R6, R7 & R12 (more drive gain and less output gain).
-Changed U2 to an OPA2604, because it can drive 600 ohm loads.
-Changed the 4558 opamp with an OPA2134 (FET front end).
-Removed R24 across the primary coil of the tranny, because more current was needed to drive the primary coil harder for more core saturation.

After the changes:

*The output amplifier will not clip anymore.
*The U2 primary driver now clips at -5.6V instead of -4.2V… an increase in headroom of 1.4V.
*The input signal at U1a pin 3 is rock solid (no bias drifting observed).

With the new changes, I was able to measure an increase in THD of 1.5% at the output, with an input signal of 40Hz and the drive adjusted to 12Vp-p across the primary coil. That is 12mA(peek to peek) thru the primary coil and this tranny is supposed to start to core saturate at 4mA.

It is too late for listening tests, but I will post my listening observations tomorrow. Hopefully I will be able to hear the LF transformer core saturation.

-Frank

 

[johnk_10]

cool! I can't wait to hear the listening test results.

 

[boomertech]

cool! I can't wait to hear the listening test results.

John, you have to try the new changes! The range of tones now available has greatly improved.

Listening tests with Schecter C5 bass:

With the clip control at minimum I increased the drive control until I just heard distortion on the attack of each note and then backed off the drive a little bit. This sounds very nice and rich in a clean way, where the initial attack of each note jumps out to tap you on the forehead. The mid frequencies seem to be pulled forward and the tone is very smooth. If I were still in a band, this would be my always on setting.

While thumping on the open E string, I had the clip control at minimum and adjusted the drive to just hit distortion on the initial attack of each note and let it ring out. I was trying to see if I could determine if the distortion that I heard was from core saturation or U2 clipping. Honestly, I couldn’t tell one way or another, because the distortion to clean transition was so quick and I don’t know if core saturation sounds like amp clipping. But the math of the circuit says that the transformer should be beginning to core saturate at this point. Maybe the difference in the mid frequencies that I was hearing was from saturation, but I cannot say for sure.

Adjusting the drive and clipping controls gave a nice range of useful distorted tones. The distortion is not jagged and raw sounding anymore. So by removing the output amplifier clipping, the range of distortion tones have improved greatly in my opinion.

I did find that heavier settings of the clip and drive controls was a bit too much for my tastes on my B string. I like my really low notes clean, so I will try a blender with this pedal in the future to see how well it mixes with the dry signal.

Listening tests with Strat guitar:

With the clip control at minimum, I kept increasing the drive higher and higher until I hit max drive. All thru that range, the tone went from very clean to moderately driven. Then I started adjusting the clip control from minimum to maximum for more grit. I just couldn’t get a bad tone out of this pedal on guitar! It sounded fantastic at all settings! Playing the lead in chimes for YES Roundabout and Rush Red Barchetta blew me away with the heavier control settings. It nails some great blues and southern rock tones… to me, it sounds like a pushed little tube amp. Just beautiful!!!

Although I am still not sure if I am hearing core saturation, I really like this little pedal. It does the pushed amp thing nicely and enhances my amplifiers’ tone beautifully. Even with over 53dB of drive gain, this circuit is exceptionally quiet. I think that if this pedal was marketed for guitar players, that John might have the next pedal to achieve magical status over on Harmony Central.

I will post the schematic with all of the new changes soon.

-Frank

 

[nshuman]

I experimented with the T-B pedal today and have a few more recommended suggestions. First I will share my observations with the circuit that we finalized on Jan 16.

The 9V supply that I used has an output of 9.34Vdc.

Original circuit (Jan 16):

U1b output amplifier clipped long before the U2 primary drivers clipped. There is just too much gain at U1b. Solution= reduce output gain for U1b. (Suggested revision on Mar 16)

U2 transformer primary drivers clipped on the negative portion of the signal at -4.2V with the OPA2134. The supply pins of U2 (pins 8 & 4) measured 9.08V and -7.61V. Solution= to increase headroom, an opamp with more output horsepower is needed.

Another thing that I noticed was the input signal at pin 3 on U1a was slightly shifting bias levels. The R1 2M2 bias resistor is too light for the BJT front end of the 4558 opamp. To keep the approx 1M ohm input impedance, this opamp will need to be changed to a FET or CMOS front end.

Changes that I made today:

-Installed the Mar 16 suggested revisions for R6, R7 & R12 (more drive gain and less output gain).
-Changed U2 to an OPA2604, because it can drive 600 ohm loads.
-Changed the 4558 opamp with an OPA2134 (FET front end).
-Removed R24 across the primary coil of the tranny, because more current was needed to drive the primary coil harder for more core saturation.

After the changes:

*The output amplifier will not clip anymore.
*The U2 primary driver now clips at -5.6V instead of -4.2V… an increase in headroom of 1.4V.
*The input signal at U1a pin 3 is rock solid (no bias drifting observed).

With the new changes, I was able to measure an increase in THD of 1.5% at the output, with an input signal of 40Hz and the drive adjusted to 12Vp-p across the primary coil. That is 12mA(peek to peek) thru the primary coil and this tranny is supposed to start to core saturate at 4mA.

It is too late for listening tests, but I will post my listening observations tomorrow. Hopefully I will be able to hear the LF transformer core saturation.

-Frank

Were those all pretty straightforward to do on the PCB version of the pedal?

 

[johnk_10]

i'll try those changes.
and BTW, if I like it better (and I probably will) , anyone that has one of my builds can send me the pedal and i'll update it at no charge.
(there's only a couple of them out there).

 

[mrwood4]

Just curious, would you be making the changes to the ones your currently building?

 

[johnk_10]

yep.

 

[boomertech]

Here are the latest suggested changes:

-Frank

 

[BuzzKing]

Here are the latest suggested changes:

View attachment 583026
-Frank

So when removing R24 do you replace with a jumper? Or does R17 go straight to pin 2 and R18 goes straight to pin 4 of the transformer?

-Rick

 

[boomertech]

Were those all pretty straightforward to do on the PCB version of the pedal?

I didn't remove the PCB, I just paralleled resistors for R6, R7 and R12 to get their values in the ballpark. Removing R24 was trickier because it is on the transformer PCB that's tucked down neatly under the edge of the main PCB. I used a hobby knife to cut one lead on R24 and positioned it out of the way. The opamps are in sockets so they were a breeze to change.

-Frank

 

[boomertech]

So when removing R24 do you replace with a jumper? Or does R17 go straight to pin 2 and R18 goes straight to pin 4 of the transformer?

-Rick

Just remove R24 from the circuit... no jumper, everything else stays the same.

-Frank

 

[schlick vicious]

The soft clipping feedback network of U2a looks pretty interesting on its own. Did you probe or listen to the signal after that inverting amplifier?

Awesome project, really nice idea!

 

[boomertech]

The soft clipping feedback network of U2a looks pretty interesting on its own. Did you probe or listen to the signal after that inverting amplifier?

Awesome project, really nice idea!

While prototyping this circuit, we bypassed the transformer until the rest of the circuit was verified. So John has listened to it several times by itself.

I have built similar circuits back in the 90's and that style of clipping produces nice round-top clipping. Music Man used this type of clipping in some of their solid-state preamp (with tube power) hybrid amplifiers in the early 1980's. It was used to tube-i-fy the preamp signal before going to the tube output power section.

-Frank

 

[johnk_10]

i just built up the newest revised version and it's frickin' awesome!
for those of you that have one of my original builds, please send it back to me and I'll update it to the latest version at no charge. if you liked the pedal before, you'll love it after the revision.
if you don't want to send it back and want to do it yourself, i'll instruct you how to do it with minimum invasive mods (i.e.you don't have to remove any of the circuit boards).

 

[nshuman]

Now if USPS/Canada Post can just get off their butts and deliver this to me

John, is it mostly just desoldering and resoldering new components? Bypasses, stuff like that?

 

[mrwood4]

i just built up the newest revised version and it's frickin' awesome!
for those of you that have one of my original builds, please send it back to me and I'll update it to the latest version at no charge. if you liked the pedal before, you'll love it after the revision.
if you don't want to send it back and want to do it yourself, i'll instruct you how to do it with minimum invasive mods (i.e.you don't have to remove any of the circuit boards).

Awesome!! can't wait to try this pedal!!

 

[johnk_10]

Now if USPS/Canada Post can just get off their butts and deliver this to me

John, is it mostly just desoldering and resoldering new components? Bypasses, stuff like that?

there's really no de- soldering components. all you have to do is cut out one resistor, and piggyback 3 others. then move the OPA2134 to where the RC4558P is (U1) and install an OPA2604 where the OPA2134 was.

 

[nshuman]

there's really no de- soldering components. all you have to do is cut out one resistor, and piggyback 3 others. then move the OPA2134 to where the RC4558P is (U1) and install an OPA2604 where the OPA2134 was.

Sounds cool. I'll see when I actually get it, I might be out of town for a while, so it may take longer to get it, even if USPS/CanPost get their act together.

 

[boomertech]

i just built up the newest revised version and it's frickin' awesome!
for those of you that have one of my original builds, please send it back to me and I'll update it to the latest version at no charge. if you liked the pedal before, you'll love it after the revision.
if you don't want to send it back and want to do it yourself, i'll instruct you how to do it with minimum invasive mods (i.e.you don't have to remove any of the circuit boards).

I thought you would like the revisions.
I played recorded music thru this pedal all day yesterday and it sounds great as a little preamp!

-Frank

 

[blindrabbit]

I thought you would like the revisions.
I played recorded music thru this pedal all day yesterday and it sounds great as a little preamp!

-Frank

Funny, I was just thinking to myself today...I wonder if this could be turned into a preamp/the front end of an amp with the addition of an EQ circuit (and depending upon how far one wanted to go with it, a DI section/amp/headphone amp/whatever). Once you mentioned that part of the circuit was similar to the front end of the old Musicman amps, I figured, well why not add some of this lovely saturated transformer tone to a nice Class D power section...