F5 power amplifier

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potstip

Member
Joined 2013
Hello;

Due to i quit my job Finally i have a time to complete my f5 amplifier. Yesterday i completed soldering and i have few questions and need help. I have v3 board btw. Please see attached pictures.

1. Do i have to solder led and led resistor or can i keep it as blank.

2. Is it fine to use 0.4w rated chardof zfoil 10ohm resistor for r3 and r4. Or i need to desolder and solder minimum 1watt.

3. Q1 N-Channel JFET Input 2SK170
Q2 P-Channel JFET Input 2SJ74
Did i soldered their direction wrong? Currently flat faces touching but i guess round faces should touch?
 

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Last edited:
Sangram

Sangram

Moderator
Joined 2002
Paid Member
potstip said:
2. Is it fine to use 0.4w rated chardof zfoil 10ohm resistor for r3 and r4. Or i need to desolder and solder minimum 1watt.
Click to expand...

The 10 ohm resistor has to handle quite a lot of power. With a 0.4W resistor you want to keep peak dissipation below about 200mW.

Actual possible Pd of part at 20Vp output is exactly 1 watt. Desired rating is therefore about 2W (at 50% Pd the case of the resistor will be between 80 and 100C, and high temperatures shorten part life).

This means with allowable max of 200mW you can have about 9Vp on output, or just about five watts of usable output power. Still enough for a high sensitivity speaker system, but you'll burn that resistor if you push the amp harder.

I've had to repair some DIY implementations that originally used a 1/4w Dale resistor in that position - every single one of them were burnt to a crisp in less than a year. The output MOSFETs and JFETs were toasted, and one poor fellow burned out his woofer coils as well.
 
M

McQuaide

Member
Joined 2011
sangram said:
The 10 ohm resistor has to handle quite a lot of power. With a 0.4W resistor you want to keep peak dissipation below about 200mW.

Actual possible Pd of part at 20Vp output is exactly 1 watt. Desired rating is therefore about 2W (at 50% Pd the case of the resistor will be between 80 and 100C, and high temperatures shorten part life).

This means with allowable max of 200mW you can have about 9Vp on output, or just about five watts of usable output power. Still enough for a high sensitivity speaker system, but you'll burn that resistor if you push the amp harder.

I've had to repair some DIY implementations that originally used a 1/4w Dale resistor in that position - every single one of them were burnt to a crisp in less than a year. The output MOSFETs and JFETs were toasted, and one poor fellow burned out his woofer coils as well.
Click to expand...


The BOM I have recommends 1W for these, and checking mine I have 1/2W in my build. There's no noticeable scorching on them but I ordered some 2W replacements to be safe.
 
P

potstip

Member
Joined 2013
Wow thanks for fast response guys.

I add 1watt ones into free slots as addition.

I am planning to put 3 different amplifier in same case so f5 is the first one jhl 2003 will be second (which is ready) and third one i did not decided yet.

I make a fast progress and completed power soft start + CLC. side today. If i have enough time i will complated low voltage cabling tomorrow.

I appreciated if have any comments to improve it.
 

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cviller

cviller

Member
Joined 2005
potstip said:
Wow thanks for fast response guys.

I add 1watt ones into free slots as addition.

I am planning to put 3 different amplifier in same case so f5 is the first one jhl 2003 will be second (which is ready) and third one i did not decided yet.

I make a fast progress and completed power soft start + CLC. side today. If i have enough time i will complated low voltage cabling tomorrow.

I appreciated if have any comments to improve it.
Click to expand...

The parallel ones you have added changes the circuit, ie two 10R in parallel gives 5R resulting in different gain.
 
M

McQuaide

Member
Joined 2011
H

Hikari1

Member
Joined 2012
potstip said:
Wow thanks for fast response guys.

I add 1watt ones into free slots as addition.

I am planning to put 3 different amplifier in same case so f5 is the first one jhl 2003 will be second (which is ready) and third one i did not decided yet.

I make a fast progress and completed power soft start + CLC. side today. If i have enough time i will complated low voltage cabling tomorrow.

I appreciated if have any comments to improve it.
Click to expand...

Adding 10R in parallel at R4 and R3 will result in 5R combined and also the wattage rating will be the same as the lowest rated resistor...so, take out the Z foils at R4 and R3.

Also, you have used your Z foils mostly on the current limiting portion of the amplifier...nothing wrong with that but those resistors will never impact audio quality unless the current limiting activates, which won't sound good anyways.

Critical resistor positions are R1, R3/R4, R13/14 (you have Z foils here) and feedback and source resistors which need 3 watt resistors, so no z foils there.
 
H

Hikari1

Member
Joined 2012
labjr said:
I was told that low value resistors don't have much impact on the sound quality. Using Z-foils may have no real benefit in this circuit.
Click to expand...

It totally depends on the function of the resistor in the circuit...not the value. And in a simple, low-parts count amplifier like this every resistor (with the exception of the current limiting circuit) is doing something important.
 
L

labjr

Member
Joined 2003
Hikari1 said:
It totally depends on the function of the resistor in the circuit...not the value. And in a simple, low-parts count amplifier like this every resistor (with the exception of the current limiting circuit) is doing something important.
Click to expand...

I was going to change the resistors in my SET amps to Z-foils which are very simple. The designer (who is very reputable) told me not to spend the money.
 
H

Hikari1

Member
Joined 2012
labjr said:
I was going to change the resistors in my SET amps to Z-foils which are very simple. The designer (who is very reputable) told me not to spend the money.
Click to expand...

“Worth the money” is subjective and depends on how much money you have. Tube amp guys routinely put carbon comp resistors in amps to “flavor” them.

I have never used z foils. PRP and Dale are good enough for me. If you want to throw money at an F5 I think caps and heatsinks are the two best places. Or Toshiba outputs.
 
L

labjr

Member
Joined 2003
Hikari1 said:
“Worth the money” is subjective and depends on how much money you have. Tube amp guys routinely put carbon comp resistors in amps to “flavor” them.
Click to expand...

Depends on the value. I wasn't trying to say it wasn't worth the money. Just that I was told low resistor values make hardly any difference in the sound.

I have never used z foils. PRP and Dale are good enough for me. If you want to throw money at an F5 I think caps and heatsinks are the two best places. Or Toshiba outputs.
Click to expand...

I thought Nelson preferred the Fairchild outputs in the F5?
 
H

Hikari1

Member
Joined 2012
labjr said:
Depends on the value. I wasn't trying to say it wasn't worth the money. Just that I was told low resistor values make hardly any difference in the sound.



I thought Nelson preferred the Fairchild outputs in the F5?
Click to expand...

I think Nelson preferred to use mosfets that were being manufactured and available at the time, which the Fairchild’s were.

Now they are out of production so IRFs are the common choice.

The Toshiba vertical mosfets are no longer in production, expensive and hard to find.

I have used all 3 types. I think I prefer the Toshiba but obviously it’s impossible to A/B test them at the same time. But the Toshibas were the most expensive, so they must be the best! That’s a joke. Kind of.

The XA series uses IRF parts.
 
P

potstip

Member
Joined 2013
Thanks Hikari1 and cviller i removed 10ohm zfoils.

I am planing to bias it today. Bias procedure starts with.
"Please make absolutely sure that all variable resistors P1 and P2 are turned fully counter-clockwise."

But direction is chande depends on which side you look the board. The one that i showed on the picture is correct?
 

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