F5 power amplifier

Jump to Latest
Franz Gysi

Franz Gysi

Member
Joined 2007
AndrewT said:
use your scope to look at the HF roll-off.
use a 1kHz sinewave that will result in ~2 to 3Vac at the output of the amplifier, with a 100r resistive load.
Set up one channel with the input signal at near full height of the screen.
Set up the other channel to monitor the output. Set it up to match the height of the input trace.
Now start increasing the sinewave test frequency.
It does not matter if the voltage of the sinewave varies a bit as you change frequency.
Now look at the difference between the output waveform and the input waveform.
If the frequency response is exactly -0dB and the phase is also exactly 0degrees then the output will lie exactly on top of the input trace.

If the frequency response drops by 0.5dB the output signal will be noticeably different from the input signal. A 6% drop in relative trace heights is -0.5dB. You can see that on the screen.

If you get to 100kHz and still can't see the output signal dropping relative to the input signal then that tells you the bandwidth is wider than 100kHz.

If the difference between the signals is ~12% (output = 89% of input), then that is -1dB
If the output appears to fall to 71% of the input height then you are at the F-3db frequency.
Click to expand...

Andrew

I am sorry, but I dont have a two channel or 100kHz scope.

So, I prefer not to make any more measurements actually and compare later with the other MOSFET's.

On my version of your board my "Q1" is in the complete opposite direction to yours, and mine sounds brilliant, even tube lovers with McIntosh valve gear love it.
Click to expand...

Indeed, they work fine. I also built one stereo "full" F5 and it is perfect. The orientation of a JFET is absolutely irrelevant as source and drain are equivalent.

I also don't like to buy pirat products and the seller knows exactly, why he is not showing the silkscreen side of the boards in his advertisements.

Again, Nelson: Sorry!

BTW: when I am importing fake copies of swiss watches from asia, customs can confiscate it...

Franz
 
SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
awc said:
But the world wide publicity they give to Pass Labs / First Watt, I would think Mr Pass makes up for that way, and the stamp "PASS F5" on them dose just that.
What you lose on the swings you pick up on the roundabout.:)
Click to expand...

No. It's theft and lying, plain and simple. It is immoral and anyone dealing with this vendor is supporting criminal activity and encouraging further theft.
 
Blues

Blues

Member
Joined 2003
Btw, I don't have an F5 built...:)

This is due to the 100//100 feedback resistors in parallel to the 10R Source resistor which in turn are in parallel with the high R of the jfet Gate-Source plus the 100k Gate resistor to Ground. There are 2 of these feedback loops in parallel.

Set it to just the Ohm function on your DMM so you can get a reading...
 
Tripmaster

Tripmaster

Member
Joined 2008
Blues said:
Btw, I don't have an F5 built...:)

This is due to the 100//100 feedback resistors in parallel to the 10R Source resistor which in turn are in parallel with the high R of the jfet Gate-Source plus the 100k Gate resistor to Ground. There are 2 of these feedback loops in parallel.

Set it to just the Ohm function on your DMM so you can get a reading...
Click to expand...

Hello again

Both read 30R between +&-
 
Tripmaster

Tripmaster

Member
Joined 2008
Blues said:
There's more than meets the eye...and it looks like the low R of the jfets D-S (~30R to 50R) and the high R of the mosfets D-S are also in play. Try to measure R from +Out to jfet D...what is it?
Click to expand...


Right Channel

SPK Out +/- =30.1R
R5,6,7,8 (100R) =29.3R in circuit
Q1 (2SK170) =66.8R between SPK + and JFET drain
Q2 (2SJ74) =29.3R between SPK + and JFET drain


Left Channel

SPK Out +/- =30.1R
R5,6,7,8 (100R) =29.3R in circuit
Q1 (2SK170) =67R between SPK + and JFET drain
Q2 (2SJ74) =29.3R between SPK + and JFET drain
 
Last edited:
Franz Gysi

Franz Gysi

Member
Joined 2007
Franz Gysi said:
Here how the 1kHz square wave looks like (on my old Heathkit OS2) with the actual, unsatisfiying version of my Mini F5

An externally hosted image should be here but it was not working when we last tested it.


Too restricted bandwith.

Franz
Click to expand...

Please forget about this image!

I just connected my old signal generator directly to my old scope, and it is looking nearby the same :yell:

I tried also with direct connection the sine wave with increasing frequency, but I get decreasing output from the signal generator significantly over 8kHz. Some revision needed...

So, I am sorry: with this equipment, I will not be able to measure bandwidth.

Franz
 
Tripmaster

Tripmaster

Member
Joined 2008
sangram said:
@tripmaster: Your readings seem completely in line with mine. Two 60 ohm resistors in parallel between output and gnd, look at the schematic :)
Click to expand...

I get it :)

Are there any additional readings I can take to check the amp is operating to spec?

I intend to re-check the bias as its been a few weeks since I built the amp, and the ambient temperature was much higher than it is now.
 
Mooly

Mooly

Administrator
Joined 2007
Paid Member
Franz Gysi said:
So, I am sorry: with this equipment, I will not be able to measure bandwidth.

Franz
Click to expand...

As long as the 'scope is OK you can :)

1. Set the amp and generator so you have say 10 volts peak to peak at the amp output using a low frequency of say 400 to 1000 hz.

2. Now see what input level that needs... in other words what is the generator output.

3. Now increase the frequency to 10kHz and again adjust the generator so that it is the same output level (generator not amp) as in 2 above.

4. Now measure the output of the amp... it should still be 10 volts peak to peak or whatever value you decided to use.

5. Keep increasing the frequency until the amp output voltage level falls to around 7 volts peak to peak. That is the "half power" point or -3db point.
 
Blues

Blues

Member
Joined 2003
sangram said:
@... Two 60 ohm resistors in parallel between output and gnd, look at the schematic :)
Click to expand...

Tripmaster said:
I get it :)...
Click to expand...

Of course there's more to it than that although it just happens the values (feedback+Rs 2x in parallel) add up to the actual reading. The 29R in-circuit that Tripmaster measured on the feedback resistors tells the story of the other loops that are involved.

The 67R from +Out to the Njfet Drain and the 29R from +Out to Pjfet Drain form a parallel loop which equals about 20R. This in turn adds to the 10R Rs either way to Ground to give us the 30R.
 
Tripmaster

Tripmaster

Member
Joined 2008
Blues said:
Of course there's more to it than that although it just happens the values (feedback+Rs 2x in parallel) add up to the actual reading. The 29R in-circuit that Tripmaster measured on the feedback resistors tells the story of the other loops that are involved.

The 67R from +Out to the Njfet Drain and the 29R from +Out to Pjfet Drain form a parallel loop which equals about 20R. This in turn adds to the 10R Rs either way to Ground to give us the 30R.
Click to expand...

Hi Blues

So are you saying something doesn't look right with my measurements?
 
You must log in or register to reply here.
Top Bottom