Franz, you are on the way back to the original F5. Are you still making a Mini F5 ?
Yes, still a Mini F5!
The restriction to Mini is due to limited dissipation of the heathsinks and the limited power of the transformers.
The listening tests are quite satisfying now. Maybe still a little bit glare in the highs missing.
I will freeze it now for some days as I am moving my workshop. As soon as I will have the testequipment accessible, I will check the Mini F5.
Franz
The restriction to Mini is due to limited dissipation of the heathsinks and the limited power of the transformers.
The listening tests are quite satisfying now. Maybe still a little bit glare in the highs missing.
I will freeze it now for some days as I am moving my workshop. As soon as I will have the testequipment accessible, I will check the Mini F5.
Franz
I've read a little bit more about mosfets and hexfets. I also think to understand the F5 circuit better than ever before 
Now, I am quite sure, in my Mini F5 the voltage gain stage has to drive high gate capacitances of the IRFP's. So, I could try 2SK1529/SJ200 like mentionned in this thread to optain reduced capacitances
But I could also try to get higher current and lower output impedance from the gain stage.
So, I decided to reduce R1 and R2 by paralleling 15R resistors, resulting in 6 Ohm.
For my understanding, this does result in a slightly higher current through the input stage and a higher gain.
It also reduces the feedback by some dB's.
And it seems to "open" the sound.
Now my question:
I got 2SK170/2SJ74 with Idss of 6.8mA.
Is it recommended to use higher Idss devices?
Franz
Now, I am quite sure, in my Mini F5 the voltage gain stage has to drive high gate capacitances of the IRFP's. So, I could try 2SK1529/SJ200 like mentionned in this thread to optain reduced capacitances
But I could also try to get higher current and lower output impedance from the gain stage.
So, I decided to reduce R1 and R2 by paralleling 15R resistors, resulting in 6 Ohm.
For my understanding, this does result in a slightly higher current through the input stage and a higher gain.
It also reduces the feedback by some dB's.
And it seems to "open" the sound.
Now my question:
I got 2SK170/2SJ74 with Idss of 6.8mA.
Is it recommended to use higher Idss devices?
Franz
Preamp for F5
juma:
I have built your J310 JFET preamp(from the JFET BOZ thread) for the F5. It is good. The gain is about 2+ times, which is what I want for F5 and other amps. Id of both J310 are about 10mA. I've also used your capacitance multiplier and LM317 for regualtion. However, I can hear a little high pitch hiss noise when I press my ear close to the speakers. Is that normal?
Elec.
juma:
I have built your J310 JFET preamp(from the JFET BOZ thread) for the F5. It is good. The gain is about 2+ times, which is what I want for F5 and other amps. Id of both J310 are about 10mA. I've also used your capacitance multiplier and LM317 for regualtion. However, I can hear a little high pitch hiss noise when I press my ear close to the speakers. Is that normal?
Elec.
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Oh yes, I have overseen that. It is not a source follower.
So, my conclusion:
Driving the F5 as Mini F5 (reduced voltage and idle current) does result in high capacitances at the gates of the power transistors.
Too high to be driven correctly by the first stage.
Is the only solution to use 2SK1529/2SJ200 ?
Franz
If I were you, I'd run mini-F5 at +/-15V with two pairs of 2sk2013/2sj313 in output (each channel) biased at 400-500mA through each pair. The capacitancies are much lower and the linearity of those parts is significantly better. The resulting sound is noticeably better than standard F5.
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I have a question what is the more important to "quality of sound " 000 on speakers 0.600 on R11 R12 OR the 5 seconds hold hand on heat sink.?
it appears to me running it hot bias 750 creates a deeper base that lets music come through at low volume eg "first watt" idea But not certain accurate sound.
any thoughts ? if wrong place fell free to shift it.
it appears to me running it hot bias 750 creates a deeper base that lets music come through at low volume eg "first watt" idea But not certain accurate sound.
any thoughts ? if wrong place fell free to shift it.
most parameters of semiconductors change with temperature.
If the designer chooses to use a particular "hot" parameter to achieve a design goal then the builder must follow suit.
However, changes in temperature can cause quite random changes in semiconductor performance.
The lower the range of temperature the smaller will be the size of the random performance change.
Keep all the devices cooler and the performance should be more consistent than allowing the temperatures to vary a lot.
If that makes any sense then 50degC on a heatsink is of no relevance to the sound quality coming out of a speaker. But it is likely to give more consistent performance than a 70Cdegrees temperature rise in the heatsink.
Similarly if the range of temperature could be reduced even more than allowing the worst temperature to reach 50degC then I would aim for that on the basis of consistency of performance. 45degC might sound better over a bigger range of operating conditions than 50degC over a similar range of operating conditions. More consistency rather than better absolute quality.
What this comes down to is that ClassA output current capability is directly attributable to sound quality from the speaker. Heatsink temperature is a secondary effect. Use a bigger heatsink if necessary.
If the designer chooses to use a particular "hot" parameter to achieve a design goal then the builder must follow suit.
However, changes in temperature can cause quite random changes in semiconductor performance.
The lower the range of temperature the smaller will be the size of the random performance change.
Keep all the devices cooler and the performance should be more consistent than allowing the temperatures to vary a lot.
If that makes any sense then 50degC on a heatsink is of no relevance to the sound quality coming out of a speaker. But it is likely to give more consistent performance than a 70Cdegrees temperature rise in the heatsink.
Similarly if the range of temperature could be reduced even more than allowing the worst temperature to reach 50degC then I would aim for that on the basis of consistency of performance. 45degC might sound better over a bigger range of operating conditions than 50degC over a similar range of operating conditions. More consistency rather than better absolute quality.
What this comes down to is that ClassA output current capability is directly attributable to sound quality from the speaker. Heatsink temperature is a secondary effect. Use a bigger heatsink if necessary.
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Hi
I have a slightly annoying hum coming from my speakers. If I place my ear next to the amp the hum from the transformer is at the same level. If I short the RCAs, the speakers are silent. My transformer is rated at 230V on a 230-250V mains supply.
The mains is currently 231V
I have placed three CL60s in series with the mains live and it doesn't make any difference.
Do you think this a simple grounding issue, noisy transformer, or would a DC trap be beneficial?
Your guidance would be greatly appreciated
I have a slightly annoying hum coming from my speakers. If I place my ear next to the amp the hum from the transformer is at the same level. If I short the RCAs, the speakers are silent. My transformer is rated at 230V on a 230-250V mains supply.
The mains is currently 231V
I have placed three CL60s in series with the mains live and it doesn't make any difference.
Do you think this a simple grounding issue, noisy transformer, or would a DC trap be beneficial?
Your guidance would be greatly appreciated
Less feedback makes the sound more relaxed to me, which
you may translate as open. I like it that way myself.
Since you are reducing the resistance on the Sources of the
Jfets, it's probably wise to stick with 6.8 mA Idss, which will
keep the temperature down on them.
Hi Trip,
shorting the amp input reduces speaker hum to inaudible. Does the transformer noise change?
What is the measured noise coming from the terminals.
Adding a source brings back the speaker hum. This could be hum coming from the source or it could be induced by the connections between the units.
Have you tried connecting different sources?
You may find that only one or few sources cause the hum problem. Which?
shorting the amp input reduces speaker hum to inaudible. Does the transformer noise change?
What is the measured noise coming from the terminals.
Adding a source brings back the speaker hum. This could be hum coming from the source or it could be induced by the connections between the units.
Have you tried connecting different sources?
You may find that only one or few sources cause the hum problem. Which?
Hi
I have a slightly annoying hum coming from my speakers. If I place my ear next to the amp the hum from the transformer is at the same level. If I short the RCAs, the speakers are silent. My transformer is rated at 230V on a 230-250V mains supply.
The mains is currently 231V
I have placed three CL60s in series with the mains live and it doesn't make any difference.
Do you think this a simple grounding issue, noisy transformer, or would a DC trap be beneficial?
Your guidance would be greatly appreciated
I had also a small buzz, my solution was a ground star plus grounding the shield of the rca cable (both sides,together with the one of the two inner cables, it was a balanced shielded cable)
http://www.diyaudio.com/forums/atta...73-f5-power-amplifier-firstwatt-f5-inside.jpg
