Quote from: aquataur on June 27, 2026, 12:13:33 PMHe explains in depth that the balance does not mean symmetric signals, but balanced impedances.

Indeed, balanced transmission works even if you send signal only down one of the two equal-impedance conductors. (Of course, you must avoid sending identical signal of the same polarity down both.)

The noise is what is counted on to be symmetric: but then both lines have to face identical impedances, so that the noise is subject to identical transfer functions.

If identical but noises are filtered differently, they are no longer identical and do not subtract to zero.
 

Quote from: branko_76 on June 12, 2026, 12:08:49 AMI bought a used Ampeg J-20 (20 watts 6V6 PP w/ 12" speaker) tube amp last year and was never happy with its thin and boxy sound.

Last month I bought another Ampeg, this time a solid state GT-10 (20 watts w/ original 10" CTS speaker). Comparing it to the J-20, it sounded much more full with a better low-end.

The previous owner of the J-20 swapped the original alnico 12" speaker for an Eminence Wizard so I tried a couple of vintage Alnico 12" JBL's, D131 & D123. They both sounded better than the Eminence but not nearly as good as the GT-10 with 10" speaker.

Since the previous owner (and myself) had mauled the baffle by swapping speakers, I decided to make a new baffle and cut a hole for a 10" speaker instead of the factory 12" which seemed to crowd the cabinet. I used threaded inserts instead of tapping the screws directly to the plywood as the factory did, it's much more secure and allows me to easily swap speakers in the future if the need arises.

This was a good move, the 10" Celestion is a very good match for this amp and enclosure, it sounds as good and maybe better than the solid state GT-10...

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Way to go 
Amazed at the 10" sounding fuller and better than a good 12" but hey 

Hi,
thanks for the great write-up.
I particularly like the section on ground planes, because people splash ground planes over all PCBs, thinking that this relieves them from a thoughtful routing.

For balanced inputs (and many other hum related subjects), there is a paper by Bill Whitlock from Jensen Transformers called Design of High-Performance Balanced Audio Interfaces, hosted by Rod Elliott. He explains in depth that the balance does not mean symmetric signals, but balanced impedances.
It would nicely fit your write-up.

You may also be interested in:

• Power and Grounding for Audio and Video Systems
  A White Paper for the Real World – International Version, Jim Brown, Audio Systems Group, Inc.
• TUTORIAL 5450 (Maxim) Successful PCB Grounding with Mixed-Signal Chips -
  Follow the Path of Least Impedance, By: Mark Fortunato
• A practical guide to high speed printed-circuit-board layout, by John Ardizzoni (Analog.com)

If anybody thinks - I only do audio stuff, that does not concern me... let me correct that: audio amplifiers love to oscillate in the MHz region. Once at least.
It is really a rabbit hole you are opening up here, but for serious design, it is inevitable to dive into.
I guess all amp designers did so.

Hi everyone,

I'm new here, pleased to join this forum! I'm picking my audio electronics hobby back up again after putting it aside for a really long time.

https://electrucio.substack.com/p/ground-loops

I would like to share a compilation of ideas I did about this topic, I hope it is useful for someone.

After almost a year, I have updates.
First of all, my apologies for being away so long, life got in the way and I had to push the project aside.

That said, the updates are:

- I ditched the battery-powered idea, removed the holder, and closed up the holes I had made when installing them (3D printing pen filament and a lighter is a surprisingly effective combo);

- I figured out most of my problems come from the amp being REALLY sensitive. Apparently the mono module board is preset from factory at 36dB for the BTL configuration, giving it an input sensitivity so small you can drive the amp into distortion by just plugging your instrument directly into it;

- I've tested the amp now with both the internal 8 ohms 25w speaker (I mistyped in the first post as it being 30w btw), and an external 4 ohms 25w speaker, at both 12v and 19.5v, and had no issues with it cutting out even when using my Joyo Bass headphone amp pushing it at full volume. It did distort on both speakers and both PSUs, but did take more signal to get there at 19.5v;

- added a series 3u3 50v capacitor with the tweeter, and it helped reduce some distorting on the trebles from it being driven too hard;

Right now, I'm reconsidering what I should do regarding the preamp. Putting the Lead/Bass 12 aside for now, and I'm actually thinking of something more like, perhaps, a Honeytone's preamp, just with a tone stack replacing the single tone control. I do enjoy how those little beasts sound.

[Edit] considering that on average, at 12v on 8 ohms, the amp can put out around a 10v swing undistorted, at a gain of 36dB, the input sensitivity would be right around 150mV. The datasheet's solution is just a resistive voltage divider.
I'm tempted to just use a clipper...

Quote from: Kaz Kylheku on June 19, 2026, 02:19:06 PMHave you thought about a dual supply version, DC coupled to speaker?

It's the simplicity of a single Voltage gain stage without global feedback that makes the amp sound more like a tube amp. Cap coupled outputs just sound better. DC coupled outputs are why transistor amps sound bad.

MEF is back. Check this thread:
https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/49294-

Quote from: Loudthud on June 19, 2026, 02:31:20 AMAlso, this schematic has no short circuit protection so shorting the output could damage the output transistors. I have newer versions but they are a little more complicated.

Have you thought about a dual supply version, DC coupled to speaker?

.asc file:


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Hi all,

I was looking at some power amp schematics recently and stumbled on the Randall G3 series schematic.

I noticed that instead of current feedback, it has a fixed, four band equalizer (five, but with the fifth band indicated in the schematic with unpopulated parts).

The first and third bands of the EQ try to simulate the familiar speaker-impedance curve EQ we see when the output impedance of the amp is raised. The choice of resonant peak is a bit high at around 125 Hz; not where I would want it. The mids scoop at around 1.3 kHz.

The second and fourth bands create two closely-spaced narrow cuts at around 190 and 220 Hz. Like they are trying to cut some cabinet or room resonances, or just trying ot balance the resonant peak or something.

Well, there you go.

I will put the .asc file in a reply post.

Quote from: joecool85 on June 18, 2026, 01:44:30 PMWhat kind of output would you expect into a 4, 8, or 16 ohm load from this?

Because there is no global AC feedback, output depends on the beta of the output transistors and the resistors in the split bootstrap current source. both are 2.2K in the schematic. That value was chosen to give an output impedance of 5 Ohms with the pair of output transistors I had on hand IIRC. With a 50V rail that gives about 50 Watts (+/- 20V) into 4 Ohm load or about half that into 8 Ohms. The high output impedance makes the amp act more like a tube amp with no feedback like the famous 5E3.

This isn't a very good amp for a novice builder because you really need an oscilloscope to tweek it up and get it running correctly. Without a scope, you need to try different resistors for the 2.2Ks, somewhere between 1K and 4.7K until you get it sounding the way you want.

Also, this schematic has no short circuit protection so shorting the output could damage the output transistors. I have newer versions but they are a little more complicated.