Amp Design Myths and Truths

I would like to address some topics that have been hot on the internet lately, namely, PCB vs. PTP wiring, output transformers and line outs.

*More information about technical vocabulary can be found at the bottom.

 

PRINTED CIRCUIT BOARD AMP DESIGN
VERSUS
POINT TO POINT HAND WIRED AMP DESIGN

Both designs have benefits as well as concerns that need to be addressed. Let’s talk pros and cons first.

PTP Design
Pros

  1. Sturdy build
  2. Easily modded
  3. Shortest path between points
  4. Crosstalk* and stray capacitance* can be kept to a minimum (yes, PTP wired amps can have stray capacitance and crosstalk too)
  5. Easier to replace bad parts

Cons

  1. Expensive
  2. Poor design can lead to hum and odd problems
  3. Poor craftsmanship can lead to hard-to-fix problems

PCB Design
Pros

  1. Affordable
  2. Better uniform performance from amp to amp
  3. Better solder joints
  4. Crosstalk and stray capacitance can be kept to a minimum

Cons

  1. Requires proper design to be noise free
  2. Parts that overheat can damage the board
  3. Requires some skill to replace parts

What a builder can do to address the cons in PTP design:

  1. Take care in running all audio wires perpendicular to DC voltage and heater voltage wiring to prevent crosstalk
  2. Use a star grounding* system to minimize hum
  3. Practice stringent quality control to maintain craftsmanship

What a builder can do to address the cons in PCB design:

  1. Have solder runs only on one side of the board to prevent stray capacitances
  2. Use ground plains to shield from crosstalk
  3. Use higher wattage components to prevent heat damage

In brief:

PCB is used in pro audio gear, effect pedals, and most likely, your home stereo. Proper PCB design allows for a less expensive build without sacrificing tone. Stray capacitance and, to a lesser extent, stray inductances* are primarily a concern in high frequency circuitry and, to a much lesser extent, audio frequencies. Proper design eliminates this problem.

In PCB design the ground plane can act as a shield where sensitive signals cross; this is done in PCB design, but cannot be done in hand-wired PTP. The size of the solder trace is engineered to provide minimum resistance, just as choosing the right gauge wire in PTP.

Potentiometers mounted directly to a circuit board perform better than wired potentiometers because there is less chance of cross interference. There are absolutely no reliability issues associated with properly-installed PCB mounted pots.

In conclusion it can be said and proven that a PCB design can sound every bit as good as a PTP design.

 

OUTPUT TRANSFORMERS

Anyone who has read the information on my website knows where I stand on boutique parts, including output transformers; I would like to elaborate here.

Vintage amps sound the way they do, due to the parts and circuit design. One of the more important parts is the output transformer. Vintage amps used OT’s that are designed to handle the power consumption and no more; this design choice is due in large part to the cost and weight of OT’s. Would Fender have used a larger OT in their Champ and Bassman amps if cost were not an issue? One can only speculate, but whether or not cost was the primary factor is moot; the reasoning really does not matter. Why, you ask? Well because we like the tone of the Champ, the Bassman, the Kalamazoo and the others. The Fender Champ is the most cloned amp in the harp world; why does everyone clone the amp if the vintage models do not sound good?

So, am I saying that a large OT sounds bad? No, not at all, I am saying that it does not have vintage tone. A large OT can add bass response (if your amp needs it) and it will produce a cleaner, less distorted tone. The result can be good; tone is subjective, and if this is the tone you are looking for then definitely give it a try. If you want vintage tone, a 5-watt amp should have a smaller 5 to 8 watt OT as a saturated OT is an integral part of vintage tone.

 

LINE OUTS

There are two methods of obtaining the source for a line out: one is from the preamp and the other is from the speaker out jack. You will most likely never hear a preamp line out recommended for harp because this source does not capture the sound from the power tube section or the output transformer. By far, the better choice is from the speaker line out jack because you will capture the distortion from the power section; this source is as close as you can get to the sound you hear from the speaker.

Here is an example:

Line Out

Will you hear the effect of you speaker? No, I do not believe you will. Why not? Because it is not possible for the effects of the speaker to couple to the speaker wiring and then to the line out jack. The speaker is a mechanical device and all of the effects of the speaker are projected forward through sound pressure. Below, I will show you how the sound out of a line out jack that is wired directly off of the speaker jack is different than the sound produced by the speaker.

This is what a 500hz tone looks like with the oscilloscope connected across the line out of an overdriven amp:

wspeaker1 (2)

This is what the same signal looks like when the speaker is replaced with a dummy load:

wospeaker1 (2)

The images are almost identical. There is a slight increase in level, which I believe is due to the dummy load being a purely resistive load while a speaker is not. This is the signal that goes to the PA; if the signal to the PA is unchanged when the speaker is disconnected, then the sound from the PA is unchanged and there is no transfer of speaker tone to the Line Out.

I encourage everyone to do their research; with the world wide web anyone can make unsubstantiated claims.

 

*Technical vocabulary information can be found here:
Star grounding
Stray capacitance
Inductance
Crosstalk

 

About myself:
I am Randall (Randy) Landry. I completed a 2 year Electronic Technician course at a vocational school in 1980. Since that time I have worked in audio and digital communications for a period of 25 years. Following that, I have operated Lone Wolf Blues Co for the last 8 years, developing the finest in effect pedals and amplifiers for harmonica players.

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