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Old 11th April 2012, 08:14 AM   #1
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Default F5 with BJT at input

Hi,

never heard about F5 with BJT input until Lazy Cat published his TSSA.Other DIY member Juma had his own way to do F5 with BJT input too. (atached picture)
or post #14 TSSA - The Simplest Symmetrical Amplifier
DIY member Jay said between these two Juma's version simulated better distortion wise-typical F5 topology.Also he thinks that BJT MAY sound better than JFET in this place.

Having this information i built Juma's F5 one test channel.And it's amazing.So lively and punchy on bass.I think everyone should try it, at least those who dont have quite hard to get JFETs or those who want to experiment.Schematic is very similar to org F5 (with little changes same PCB can be used)

Personally i've never heard original F5 in my system ,so better,worse or how different is BJT versus JFET at input this is the question we will need to answer in this thread.
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File Type: gif F5-BJT-in.gif (7.2 KB, 971 views)
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Old 11th April 2012, 10:25 AM   #2
juma is offline juma  Germany
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Daudio25 asked me some questions through PM and I suggested we can discuss them here:

Quote:
Originally Posted by daudio25 View Post
1.i have 2sk1529/2sj200 mosfets. It's worth to change IRFP? If so , what do i need to change in schematics?

2.How You compare it to origininal F5? Yours BJT is better? distortion which is lower?

3.Standart FET F5 have little gain.With BJT i can drive my speaker very hard.Maybe BJT have more gain?
1.EUVL found 2sk1529/2sj200 to be better than IRFP HEXFETs (you can search for data in this forum's section). They have lower Vgs so nothing should be changed, you'll just have to readjust the bias after changing the output devices.
My choice was 2sk2013/2sj313. The thread about it is here:
F5 with 2SK2013/2SJ313
The same thing goes here too - after changing the output stage, readjust the bias.

2. I like BJT-input version too ...

3. I designed the BJT-input version with same gain as original - gain is set by 750R/150R feedback network which gives same ratio as the original does (50R/10R). So it's not the amp that is louder but it seems that you have capable signal source and/or efficient speakers.
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Old 11th April 2012, 12:00 PM   #3
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by daudio25 View Post
never heard about F5 with BJT input until Lazy Cat published his TSSA.Other DIY member Juma had his own way to do F5 with BJT input too.
There was also some discussion of F5 with alternate input devices in Greg's thread here. Some of those ideas got recycled here, in between Joachim's designs.

IMHO, your bias method (similar to second pic below) is good for MOSFET input devices as the input stage idling current is almost independent of Vgs. OTOH, I think I prefer the LED biasing arrangement in pic one for BJTs, as it gets rid of the capacitors.

The second half of Joachim's circuit (3'rd pic below) looks like a nice solution as well.
Attached Images
File Type: gif bjt.GIF (5.9 KB, 912 views)
File Type: gif mos.GIF (6.0 KB, 892 views)
File Type: gif joachim.GIF (25.1 KB, 897 views)
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Old 30th March 2014, 05:24 AM   #4
fab is offline fab  Canada
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Hi

I have experimented with circuit in post #1 with 2SK1530/2SJ201 (one pair). However, I have 0.33 ohms source resistors and a gain of 10. It is biased at about 600ma on my bench. The damping factor I obtain is only 4 is that expected? I remember that the DF of the original F5 was a lot higher than that....The gm of the mosfet I use is supposed to be similar to the IRFP... or do I have fake mosfets with muche lower GM

Thanks

Fab
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Old 30th March 2014, 10:50 AM   #5
juma is offline juma  Germany
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Hi fab !
Yes, that's to be expected because the OLG is much lower this way (input pair is strongly degenerated) which makes the GNFB also much lower and that's the cause of relatively high Zout i.e. low DF. Also, Toshiba MOSFETs have lover gm than IRFs (that lowers the DF too) but they are more linear so it seems like a fair trade.
I found that I like how it sounds, but if you want to significantly increase the DF (about 10 times) this is what you should change (sch. from post #1 of this thread):
- R14,R15 = 3k9 0.5W
- R4, R5 = 22R 0.5W
- R11,R12 = 220R 2W
- C1,C2 = 4700uF 6.3V
- remove R2,R3 in order to extend the range of bias adjustment
- Increase the bias current to 1A or so - more Id -> more gm (Yfs)

This way the gain will be about 20dB - if you want to lower it to standard F5 gain value, increase the value of R4,R5 to 43R.

Last edited by juma; 30th March 2014 at 11:03 AM.
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Old 30th March 2014, 01:55 PM   #6
fab is offline fab  Canada
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Quote:
Originally Posted by juma View Post
Hi fab !
Yes, that's to be expected because the OLG is much lower this way (input pair is strongly degenerated) which makes the GNFB also much lower and that's the cause of relatively high Zout i.e. low DF. Also, Toshiba MOSFETs have lover gm than IRFs (that lowers the DF too) but they are more linear so it seems like a fair trade.
I found that I like how it sounds, but if you want to significantly increase the DF (about 10 times) this is what you should change (sch. from post #1 of this thread):
- R14,R15 = 3k9 0.5W
- R4, R5 = 22R 0.5W
- R11,R12 = 220R 2W
- C1,C2 = 4700uF 6.3V
- remove R2,R3 in order to extend the range of bias adjustment
- Increase the bias current to 1A or so - more Id -> more gm (Yfs)

This way the gain will be about 20dB - if you want to lower it to standard F5 gain value, increase the value of R4,R5 to 43R.
Hi Juma !

What you suggest above is what I have tried but only in simulation ( before doing physical change on my board ). I wanted to first confirm that my MOSFET were not fakes. Second I wanted to know if there was something I have overlooked on his the design works ( I know it is a very low OLG). According to my simulation I could get a DF of about 6 times more ( you indicate 10 but Rie of input BJT is not 0...). But it would be still low....
The trade off that I am not so comfortable in the value changes you suggest is to reduce the drive current for the MOSFET ( R14/R15). ..
I will try again increasing the bias current on mosfets but I remember that it also reduced the max output swing of the amp ( Rds * Id)...
I thought also about reducing the CLG to increase the feedback factor thus increasing the DF but I do not have preamps with lots of gain...
However, you indicate that even with this low damping it sonds good...I suppose better than the original F5....
The concern I have is that I want an amp punchy on the bass...
Should I continue with this project?

Thanks
Fab

Last edited by fab; 30th March 2014 at 02:05 PM.
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Old 30th March 2014, 04:02 PM   #7
juma is offline juma  Germany
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High DF is only indicating a high OLG and High GNFB - I never did listen to an amp with spectacularly high DF that sounded really good.
I even don't consider the DF as an amp's property that one should care about - it's Zout that imposes some practical limitations in the relationship between an amp and the speaker, and the Zout of about 1R is approximately the highest number that I feel comfortable with and the 0R1 is the value where, IME, the law of diminishing returns starts to kick in.
As I see it, the myth of DF is generated by hifi industry and hifi magazines in order to boost sales by announcing "another world-changing great revelation" (in other words, another pile of crap).
Of course, there are some very low Z speakers that need special care in this regard but still, it often happened that those kind of speakers sounded better with low DF amps than with some high DF amps.
Shortly, DF alone is not a deal-breaker, it's easy to test this circuit and see for yourself if it works OK for you - for me it did, although two of my speakers go as low as 3.5R impedance wise.
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Old 30th March 2014, 08:22 PM   #8
fab is offline fab  Canada
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Thanks for yours comments
Since my speakers are 4 ohms nominal and I do not know how low they can get , I will target an amp output impedance of 0,5 ohms or better. That means going from 2 ohms to 0,5 using some changes you have suggested..

By the way, it is this circuit and your "F5 meets Buzquito " amp that made me design my "F5-Juma with BJT input " which was later called " VSSA " by Lazycat. Lazycat and I had the same idea at the same time but he made it commercialized on his side....My version was also different since it used MOSFET for VAS as good option you had pointed to us at the time.... Since then I have built the "VSSA Lazycat" version by curiosity but I think I prefer mine with MOSFET VAS...

So this project here and now is a trial to make a simplification of "vssa" which could be called " USSA" .... Ultra simple Symetrical amp I think that the name alone is appealing for success ...if the sound is good of course.

Fab

Last edited by fab; 30th March 2014 at 08:25 PM.
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Old 31st March 2014, 10:48 AM   #9
AndrewT is online now AndrewT  Scotland
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Since you are considering 4ohms speaker, the first option I would consider is going to a 2pair output stage.

This increases the current capability and reduces the output impedance.
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Old 31st March 2014, 01:20 PM   #10
fab is offline fab  Canada
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Quote:
Originally Posted by AndrewT View Post
Since you are considering 4ohms speaker, the first option I would consider is going to a 2pair output stage.

This increases the current capability and reduces the output impedance.
Thanks for the recommendation. The drawback of going with 2 pairs is that I need to reduce the VGS applied to keep the same overall amp bias ( 1A) as with one pair. Thus it reduces the Rc of the first stage then the gain of the 1st stage thus reduces the OLG thus increases the output impedance ( less feedback factor)...

With 2 ohms Zout I am thinking that my MOSFET transconductance is too low thus maybe fake parts I have same MOSFET from another source so I may give it a try ....

If Daudio25 could measure its output impedance that would give me some indication of the target Zout since he seems to like the sound of its amp...

Fab
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