according to my own set (i am a purist) it is off (see my findings and measurements)
- but, with the F5-trimming pot on the source resistance installed, you can compensate for this difference
- & the drain resistor P1/2 takes care of the rest. Yes you will get equil/DC
- (but the harmonics will imho differ).
I replaced Q1/2 and bought new Q5/6 and installed them. I lit it up with the bulb tester in place and the light went on then down to a glow and R2 started burning...
I guess I'm wallowing in the mire of ignorance at this point. The circuit seemed so simple. I guess I though it wouldn't be so complicated to stuff boards and set the bias. I can't exactly get new boards at this point because I have designed my chassis around the spacing of Christians V2 boards which are no longer available.
erpiii
except right here at the diyaudio store P-F5c-2V20 - Pair of F-5c boards (Makes 2 channels; Rev 2.0) - Boards
Hi, I have almost complete my F5 build. While making the initial adjustment upon power up, i got the following measurements.
On left channel board, the voltage across R11 and R12 reads .58V with 4mV different on both resistors. Output is about 3mV.
On right channel board, the voltage across R11 and R12 reads .45V only with 4mV different on both resistors. Output is about 1mV.
As the voltage drop across the R11 and R12 is lower on one channel, will it affect performance?
I also found that the output on both channels will spike up gradually to about 80mV and back down after i turn the amp off. Is this normal?
Couldn't wait to listen to it but hope someone here can comment on the above before i blow any speakers.
On left channel board, the voltage across R11 and R12 reads .58V with 4mV different on both resistors. Output is about 3mV.
On right channel board, the voltage across R11 and R12 reads .45V only with 4mV different on both resistors. Output is about 1mV.
As the voltage drop across the R11 and R12 is lower on one channel, will it affect performance?
I also found that the output on both channels will spike up gradually to about 80mV and back down after i turn the amp off. Is this normal?
Couldn't wait to listen to it but hope someone here can comment on the above before i blow any speakers.
except right here at the diyaudio store P-F5c-2V20 - Pair of F-5c boards (Makes 2 channels; Rev 2.0) - Boards
Thanks for noticing but my boards are 190 in length and the output devices are 160 on center. The boards, while very nice, are not the same as his first V2 version and will not fit my configuration.
erpiii
I checked Q1/2 and they, according to someone else, looked okay. I replaced Q5/6 with new ones and that's when it smoked. Should I be looking at the output transistors as well?
Tea-bag
Are these the boards that are 190mm long with the outer most outputs 160 on center?
erpiii
Hey erpiii,
Are you sure you're not starting with P1 and P2 full on instead of at zero? If you are measuring across R3 and R4 and are getting zero ohms to start, you have R3 and R4 shorted out via the pot's wiper. I might be misreading your post. But, you may want to double check that you are starting out with the pots in the proper starting position.
Steve
Last edited:
I measured across r3 and r4. I could turn the pot all the way down to the click and then it reads zero, thats going clock wise. When I move the screw counter clock wise I see the reading go up... very small like 5 or six turns gets me like 7/8 ohms. Can you put the pots in wrong? I thought all you had to have was the center leg centered. Should I be reading more ohms when I check across r3/r4? Both pots are orientated the same, screws to the right, depending on how you look at the board.
erpiii
what wattage of bulb did you use?
I hope you had the output bias variable resistor turned right down to zero and that the bulb was/is <=40W.
Nothing should burn or be damaged if you do first test using this set up.
Hey erpiii,
It sounds like you're starting out with the pots in the correct position.
Steve
Hi all, would need some help from you,
I am building my second F5, so was expecting no troubles putting it to work, but unfortunately I cannot bias the fets.
I have inspected everything several times, all transistors are in their right positions and not turned around. The pots were placed in their right positions and turned down: no resistance across R3 and R4.
I have V+ and V- from the power supply and no burned resistors at power up.
The problem is that turning up the pots does not increase the bias. The output offset is constant all the time.
Unlike the first build, which used the standard circuit, in this second build I'm using Peter Daniel's PCB with Tech-Diy components for the R1 circuit, plus the caddock resistor upgrade. I'm not using the thermistors and have not connected R16 and R15. I did not connect R19 and R20, like in the R1 circuit. And also do not use P3.
Any suggestion?
What measures should I try?
Should I put R19 and R20 in place? Or maybe removing the whole protection circuit?
I am building my second F5, so was expecting no troubles putting it to work, but unfortunately I cannot bias the fets.
I have inspected everything several times, all transistors are in their right positions and not turned around. The pots were placed in their right positions and turned down: no resistance across R3 and R4.
I have V+ and V- from the power supply and no burned resistors at power up.
The problem is that turning up the pots does not increase the bias. The output offset is constant all the time.
Unlike the first build, which used the standard circuit, in this second build I'm using Peter Daniel's PCB with Tech-Diy components for the R1 circuit, plus the caddock resistor upgrade. I'm not using the thermistors and have not connected R16 and R15. I did not connect R19 and R20, like in the R1 circuit. And also do not use P3.
Any suggestion?
What measures should I try?
Should I put R19 and R20 in place? Or maybe removing the whole protection circuit?
The paralleled value of R3/P1/TH1/R15 and R4/P2/TH2/R16 from the original F5is about 600 ohms, and an average BL grade JFET will draw ~7mA. 7mA flowing across 600 ohms is the 4.2V the value that you're looking to bias the MOSFETs with.
If you've a GR grade JFET which draws, let's say 4.2 mA, the combined value of the resistors/thermistor has to be around 1,000 ohms. This should be within the range of the potentiometer.
My suggestion is to lift the source leg of the MOSFETs and measure the voltage across R3,R4. See if you can adjust it to 4.2V. If not, add a few hundred ohms from the junction of R3/P1 to the drain of Q1. Make sure (if using P3) that it is "centered" and not swung to either maximum of its rotation.
I have 4 F5 amps running, one uses GR devices with no problems.
Paapt,
why did you not connect R19, and R20 150R in original Scheme, or R17, R18 100R in Rev1 scheme?
R19 with R17 and R20 with R18, are part of divider resistors for Q5 and Q6.
Perhaps this is your problem, current limiting circuit is acting.
Last edited:
Hello all. I have a question most of you probably will laugh at but anyway: what is the penalty of biasing the F5 at lower current ? For instance 0.5 A.
It was many moons ago that I constructed a class A amp and I know it is the best principle but still I want my F5 to be in class A for low volume settings as it will not be run at high volume anyway.
It was many moons ago that I constructed a class A amp and I know it is the best principle but still I want my F5 to be in class A for low volume settings as it will not be run at high volume anyway.
Nelson has published some graphs with distortion versus idle current in his paper 'Leaving Class A'. See pages 4/5.
One could say: there is a sweet spot.
Imho this depends on your speaker. For low level (typically the first watt
), you could also determine yourself and see that 0,64 A will be sufficient. Note that the structure of harmonics differs, higher idle will give less higher harmonics.
I suggest that balancing of the feedback is crucial too - next to having the best selected pairs in transconductance available, to get a good harmonic structure.
albert