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Amplifiers:Pass Labs Amplifiers

The Project


Read this first


What were some of your goals with the design of the amp? In the end, did you achieve them? If not, where did the design fall short of your expectations?

examples: I wanted to create a class A amp that produced THD 0.00001% at 1KHz but I only achieved 0.01%


Volatge Gain15.3 dB
Sensitivity8 Ohm: Value, 4 Ohm: Value
S/N Ratio
~60 uV unweighted, 20-20 KHz
Rated Consumption180W
Idle Consumption180W
Harmonic DistortionDistortion @ 1 watt .001% to .005% @ 1 KHz
Harmonic SpectrumFFT plot/plots
Square/Step responseOscilloscope Trace
StabilityPhase margin + scope traces into capacitive loads
Output configurationsCan it be bridged? Parallel drive?
Output ImpedanceValue or plot/plots


Please attach schematics to this wiki page. You can then use the tag to display them inline. Please don't be afraid to attach Gerber files to the page as well.

Note: There is a bug in the wiki right now where you can't add attachments to a wiki after 30 minutes.

Bill of Materials

PCB Availibility

Also Group Buys listed further down on this page.

Power Transformers

Either list them in this section or attach a file to the page that people can download.

Note: If this project uses other projects as components (maybe this is a full build and uses an amplifier and PSU from other projects, please list them here).

Simulations and Analysis

Embed the images for your simulations here along with descriptions for each of them so people know what you're talking about.

Construction (Steps / Tips / Notes)

This section is incomplete and will be a bit messy unyil I get all information into it.

GB F5 Guide (pcb version 2)

F5 components, parts you use

Project Initial Setup

Suggested start up should have the PS disconnected from the amp boards.
Also use a variac or dim bulb tester or both.


dim bulb testers

Bias Setting(thanks to ZM)

dial both pots to 0 ohms ( check with ohmmeter )

place one voltmeter across PSU caps ( best between + and - of PSU) to observe max voltage of PSU

place one voltmeter at output - to observe offset

place one voltmeter across one source resistors(R11 or R12) of output mosfets ; it doesn't matter which one .

for test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed

if nothing is smelling - leave Variac at full mains ;

what's important - Iq must be very low , offset is irrelevant in this moment .

now turn one pot one turn ( assuming that you have multiturns )

then turn other pot one turn

observe Iq and offset

proceed one then second pot , again just one turn

observe Iq and offset

again one turn + one turn

now you are probably in range when you can see which pot is pulling offset in right direction - to 0 .

proceed iteratively with pots , while you set - say - 75% of desired Iq and zero offset

now - put lid on box and let it cook for a while - until yo get thermal equilibrium on heatsinks

it's best to use wire/clips to leave those voltmeters in place ;

open the lid , up bias to - say - 90% of desired one ,while maintaining offset

put lid on , let it cook

check ;

if all is OK - move voltmeters for Iq and offset to other channel and repeat procedure

use it few days at 90% of desired bias , then check and set to 100%

remember - temp. equilibrium with lid on is important

simple , isn't it ?

Project Modifications

Do you have some small modifications to suggest for the project that could improve it? Please list them and tell us why they improve the design. If this is discussed in a thread, please link the thread URL here....

Modification 1

Cross the streams!

Modification 2

Install a flux capacitor!


Review 1

Review 2


Build Threads

F5 Build by 6L6

F5 Build by CanAm Man

Planning an F5 build - some beginner questions

Have you built this project? If so, please add the URL to the thread that you started to show off your build.

Related Group Buys

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