I have been wanting to build an amplifier and a set of speakers with each of my children as a graduation present, and wanted something with low parts cost, low distortion and a great reputation, so I was very excited when I read about the new(ish) F5M. I have enjoyed developing a physical layout and circuit board suited to my project during the past few months, but I am not an electrical engineer or designer - so I created this post to solicit feedback, document a test build and ask questions. Basically, I will need some help. The amp pulls together known designs onto a single circuit board and is housed in a custom chassis. The elements used are:
(many, many thanks to the designers of all of these circuits)
The details are best seen in the attached .pdf, but here are a couple screenshots:
After considering feedback and making updates, my next step will be a test build with the 300VA shielded transformer and heatsinks pictured. One of my biggest unknowns is putting the phonostage in the same chassis as the main transformer, even with a shield, I don't know if it will "hum", but the test build should answer that. After that make final updates and build with the kids 🙂
- F5M Amplifier Circuit
- F5M Power Supply Circuit
- Relay Input Selector (similar to Salas iSelect)
- VSPS Phonostage
- Whammy Regulated Power Supply (used for phonostage)
(many, many thanks to the designers of all of these circuits)
The details are best seen in the attached .pdf, but here are a couple screenshots:
After considering feedback and making updates, my next step will be a test build with the 300VA shielded transformer and heatsinks pictured. One of my biggest unknowns is putting the phonostage in the same chassis as the main transformer, even with a shield, I don't know if it will "hum", but the test build should answer that. After that make final updates and build with the kids 🙂
Attachments
The F5m, as published, has a gain of 7.
Assume your signal source is capable of max. 2Vrms.
Maximum output is ~ +/-20V, with 24V rails.
That is when the volume is also at its maximum.
At 1W 8R output, the input signal to the F5m is at 0.4Vrms.
That is an attenuation of -14dB.
If your attenuator has -60dB full range, you would be at about 3 o'clock for the volume knob.
As an example, the F5Pi has a gain of 20, also with 24V rails.
The Pass Labs INT25 has a gain of 29/35dB.
https://www.passlabs.com/products/int-25/
Patrick
Assume your signal source is capable of max. 2Vrms.
Maximum output is ~ +/-20V, with 24V rails.
That is when the volume is also at its maximum.
At 1W 8R output, the input signal to the F5m is at 0.4Vrms.
That is an attenuation of -14dB.
If your attenuator has -60dB full range, you would be at about 3 o'clock for the volume knob.
As an example, the F5Pi has a gain of 20, also with 24V rails.
The Pass Labs INT25 has a gain of 29/35dB.
https://www.passlabs.com/products/int-25/
Patrick
Thanks for the input Patrick,
I have been using a passive preamp for so long I didn't stop to think that doing so here would not maximize the available power of the F5M. What you are saying makes sense, I just don't know how to increase the gain without adding a separate preamp gain stage. It looks like there is a nice writeup in the original F5 product manual discussing how the gain is set by the input transistors. I will spend some time reading that and hopefully come up with a starting point for a discussion of how to increase the gain. Appreciate your input!
I have been using a passive preamp for so long I didn't stop to think that doing so here would not maximize the available power of the F5M. What you are saying makes sense, I just don't know how to increase the gain without adding a separate preamp gain stage. It looks like there is a nice writeup in the original F5 product manual discussing how the gain is set by the input transistors. I will spend some time reading that and hopefully come up with a starting point for a discussion of how to increase the gain. Appreciate your input!
Increasing the gain of the F5m will reduce negative feedback and hence increase its distortion.
The simplest way to add gain without extra circuit is an input transformer.
But that will lower your input impedance by 10~16x.
For example the Jensen JT-6110 reversed has an input impedance will be 2.2k.
I rarely use transformers, but I am sure others can advise you.
Patrick
The simplest way to add gain without extra circuit is an input transformer.
But that will lower your input impedance by 10~16x.
For example the Jensen JT-6110 reversed has an input impedance will be 2.2k.
I rarely use transformers, but I am sure others can advise you.
Patrick
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After reading (and admittedly some skimming) of Nelson's various F5 write ups I am wondering if a detuned F5 Turbo V2 might be a better fit than the F5M for this amp. From the excellent write up it has 22dB gain (12.5x) so it would maximize the output with a +/-24V supply and the distortion at 1W into 8 ohms (for the 32V supplied version) is reported at .005% vs the .02% for the F5M. Would modifying the F5 Turbo V2 circuit as shown below work the way I am describing? I am looking to drop one set of the output transistors to keep the size of the sinks the same as I was planning for the F5M.
Just change the feedback resistors if you need to mess with gain, as that's why F5T has more gain than F5.
Thanks 6L6, that is helpful. The lower distortion of the F5T was another factor making me think it may be better. Is the increased distortion of the F5M just the designed in 2nd harmonic boost or is there more to it than that?
Honestly I'm not 100% sure but I'd suspect it's just Nelson adjusting / tuning to get the spectra he's after. I don't know why F5m sounds better to my ears (and I'm not alone in this opinion) than F5, but it does.
I will stick with the original plan (F5M) and give it a listen with the feedback resistors as specified and adjust them if I am wanting more gain. Appreciate your simple answers.
The F5T can have more closed loop gain because the paralleled output devices give it more open loop gain.
Taking one pair off reduces the open loop gain by about 6dB, and increases distortion by the same amount.
Some people like higher distortions, like in tube amps. I don't.
The F5/F5T has this trim pot trick at the front end.
You can use that to adjust any level of distortion you want.
It only changes even order harmonics.
But of course, only you can decide what you like best.
Patrick
Taking one pair off reduces the open loop gain by about 6dB, and increases distortion by the same amount.
Some people like higher distortions, like in tube amps. I don't.
The F5/F5T has this trim pot trick at the front end.
You can use that to adjust any level of distortion you want.
It only changes even order harmonics.
But of course, only you can decide what you like best.
Patrick
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I have slowly kept working on this project focusing on adding multiple options to achieve higher gain and conduct listening tests to decide which approach I prefer. The standard F5 has lower distortion, so I revised the board to allow building an F5 or F5m and can try using more feedback within the amplifier for increased gain, and also added an optional opamp gain section between the volume control and the F5/F5m amplifier.
I have done some searching, reading, watching tutorials etc. regarding the opamp section, but still have questions. If anyone is willing, I could use some help with the questions below:
1. Desired opamp gain is 2x (6dB), ratio of feedback resistors shown below. How much current should flow through the feedback resistors?
2. Are the bypass capacitors on the power supply needed for audio frequency applications?
3. What size bypass capacitors should be used? (I saw several recommendations of .1uF, is this good?)
Thanks
It probably doesn't mean much, but the current state of the board design is below.
I have done some searching, reading, watching tutorials etc. regarding the opamp section, but still have questions. If anyone is willing, I could use some help with the questions below:
1. Desired opamp gain is 2x (6dB), ratio of feedback resistors shown below. How much current should flow through the feedback resistors?
2. Are the bypass capacitors on the power supply needed for audio frequency applications?
3. What size bypass capacitors should be used? (I saw several recommendations of .1uF, is this good?)
Thanks
It probably doesn't mean much, but the current state of the board design is below.