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Old 23rd July 2008, 03:25 PM   #191
rellum is offline rellum  France
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Hi KL

Nice to see your interest for this amp, you've still not succeed in the quest of the best one. If it happens, tell me.
This morning I made some mods and listened to "the guitar trio" of John Mc Laughlin. Very good sounding.
But don't expect an extreme resolution, the GB150D is way better in this regard. Comparing the bass range is somewhat unfair because it's psu is bigger and mine is fully dc coupled. Mooly's amp can't be. In term of speed, impact, they are very close.
I have many different amps at home, each one having something special and it generaly reflects the way the designer is feeling the music. In case of Mooly's amp, I would say it has been designed by a "romantic engineer".


Hi Karl

My power needs are so low that even a lower psu would do the job. For the safety of my speakers, I dropped the 10K to 6.2K so as not to end up with an underbiased opamp. And yes, I'm using the inductor with 10R across it but no serial output resistor. This morning I have doubled the time constant of the integrator, reduced the input low pass to 2.2K/100p, the input cap was already a 22F. As I'm feeding the amp with a 10K ALPS pot, I've added an extra RC cell at the output of the opamp (33K/0.47F) to filter out any noise produced by it. If there is a difference, it's tiny, I can't notice.

May I reduce slightly the overall gain without any harm ? My thought is to increase the feedback shunt resistor to ground, actually 470R and replace it with 620R for a 31dB gain. A I understand it, the feedback factor should remain constant and stability shouldn't suffer.

My source is a Marantz CD14 player and Cabasse Iroise speakers, a french brand.


Best regards,
Francis
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Old 23rd July 2008, 05:56 PM   #192
Mooly is offline Mooly  United Kingdom
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Hi Francis,
I had never heard of those speakers-had to Google it, they look very good and I am sure they sound it as well. Very nice
It's interesting to hear your comment on power requirements. I always smile when I see some of the other designs using 50 pairs of whatever !! In a domestic setting you don't need it.
There is no problem altering the feedback as you suggest, it won't upset anything at all. Adding extra decoupling from the OpAmp output may not be such a good idea as a second time constant is introduced. I remember playing around with this age's ago, not on this amp, and it caused problems. I think if I remember correctly it actually caused low frequency instability, particularly if the time constant were long enough.
I listen to mainly classical stuff, and find this amp excels on this, particularly strings - solo violin etc. There is non of the harshness or unpleasantness that makes you want to turn it down or turn it off. It always seems to deliver the goods somehow, something other amps I have owned have never consistently done. And I find the imaging first rate, instruments stand in their own space in the soundstage.
It's an interesting thought that I think Doug Self once mentioned. It was that if you didn't like the sound of a "perfect" amp then the answer was to have some front end unit that could "dial in" the required type of "distortion". Erm perhaps not
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Old 24th July 2008, 05:09 AM   #193
KLe is offline KLe  Australia
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Hi Francis

Yes Greg's GB150D is very good Even so, it is interesting to look at other designs, such as Moolys. I am glad you built it, your comments are always appreciated.

Tell me, did you use the fuses in your 150D?
How much of a difference did DC'ing the NFB make. (I have probably asked you this before ... )

have a good day
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Old 24th July 2008, 07:04 AM   #194
rellum is offline rellum  France
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Hi KL

If I use the fuses ?
Yes, for safety reasons I do. I don't know why you ask. If it has to do with the sound of fuses, I dont feel concerned.
I have even add one on each rail of Mooly's amp, but they only feed the driver and output stage to keep low offset in case of melting.
The GB150D allows for DC'ing and the caps have 0V across them, not the best for a long term reliable behaviour. As long as
you have control over your source, it's not an issue to DC couple.
Mooly's amp can't be DC coupled, the singleton input transistor needs a shifting. But the nice thing is that the input and feedback caps always have 5V DC across them.
Does DC'ing enhance something ? For me it has more to do with psychological reason...

Hi Karl

You're right with the added time constant at the output of the opamp. I had to reduce it for stability.
Actually I look at your preamp. I will try to source the fets. How do you feed the gates ?
Linear pot, DAC or something else ?

Francis
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Old 24th July 2008, 08:32 AM   #195
Mooly is offline Mooly  United Kingdom
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Hi Francis,
I keep looking at the picture you posted. What are the two objects on the back of the outputs -- they look like little heatsinks.
I know you say you ommited the 0.22 ohm series output resistor, but did you keep the 0.22 ohms in the Drains of the FET's.
You have probably seen the thread that's running on output inductors. How about trying without ? When I built this I didn't pay too much attention to this, in fact up to the finished version I didn't use one at all.
The preamp. When I built the amp it was done minus remote control, but this was something I always wanted to add later. The FET's are driven from a simple logic arrangement consisting of discrete transistors. Will try and draw out what I did. The gates must go no lower than around - 8 volts and no higher than around 0.3 volts. Use a 1 Mohm and a 0.1 mfd on the gate to give a nice soft switching action.
I designed the remote board as a "drop in addition" and it worked great. The front panel switch always has "control" and so if the CD input is selected and you then press tuner on the remote, the input and LED change to tuner, and the FET's switch.
This of course means the control now doesn't point to the correct source but if the control is manually moved to a new position, the spare contacts on the switch generate a reset pulse (555 monostable) which resets the logic back to control by the front panel switch (same at power on as well) The remote receiver and PIC I had to buy as my PIC programming skills are zero. The PIC gives a "high" on one of 7 data lines corresponding to when a button is pressed on the remote. It will learn any common remote commands. These then feed into my "discrete decoder".
I even added a pot to vary the volts to the H bridge output stage. It's amazing how critical it is to get a good "speed" on the motorized volume control. Nothing worse than a pot that takes all day to get there or else whizzes round that fast you have no control
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Old 24th July 2008, 11:45 AM   #196
Mooly is offline Mooly  United Kingdom
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Found these, this is the logic for decoding the data lines from the PIC. I didn't use optos in the end as it shows, used the discrete transistor option. Notice the PSU. There is no ground reference as such. The -/+ 20 volts are from two regs I built into the main PSU from the start. This board "floats" between the rails ( not strictly true as the 20 volt supply defines where zero is ) I did it this way too avoid any ground currents being injected anywhere. Works brilliantly.
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Old 24th July 2008, 11:51 AM   #197
Mooly is offline Mooly  United Kingdom
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And this one shows the reset generator for generating a pulse every time the switch is moved and the drive arrangement ( from memory ) for the FET's. The germanium diodes clamp the drive to around 0.2 volts which is perfect.
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Old 24th July 2008, 07:29 PM   #198
rellum is offline rellum  France
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Hi Karl

What you see underneath are little heatsinks on wich are bolted the 0.22R thick film drain resistors in TO220 package. There was no room on the upper side and I paid them such an outrageously high price that I don't want to fry them if something goes wrong. I suppose they are used for cross conduction reasons.

I will try without the inductor and monitor the output.

Thanks for all those informations about your pre. I was mainly interested by your antiphase circuit. I will try to feed the gates directly with a DAC driven by a PIC. It will give something very compact and I have some little experience with DACs and PICs.


Francis
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Old 25th July 2008, 03:03 AM   #199
KLe is offline KLe  Australia
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Hi Francis

Yes, the fuses I was referring too are the one's that are part of the output, but included in the NFB circuit? I found, when I have replaced the fuses, that they do take at least 4 hours to run in and have at leaner sound to them ... have you noticed this.

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Old 25th July 2008, 04:08 AM   #200
lineup is offline lineup  Sweden
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Quote:
Originally posted by Mooly
Hi Lineup

Used to play around with those CMOS chips a lot. In the configuration I use they would probably work well.

Years ago I used some others, DG308 ?
Yes, I recognize & remember DG308 hi-fi analog switches.
Quite expensive .. but very good.

There are more 'modern' such hi-fi quality analog switch chips around now. But generally, if they have very low distortion, like your JFET setup Mooly, you will have to pay for it.

Another option is of course low level REED RELAY.
These are my favourites
I have seen them used in Ultra Hi-Fi Pre Amplifier Projects!!!!

Click the image to open in full size.
Quote:
Manufacturer: SRC Devices

Reed relays in a DIL- (dual-in-line) design. The relays require a small PCB surface, have a long life and are easy to operate.
They are epoxy cast in a DIL package in a 14 pin size and can be mounted automatically, wave soldered and washed.

PRME, PRMA

Relays for low and medium loads where a long life and high reliability are important.
The relays are built up around Clare's reed switch DYADTM which has ruthenium coated twin contacts
and a guaranteed load from 0 level, dry-circuit, to 10 VA.
Original package: tubes of 25 pcs.
This model is interesting. Single in line, SIL, pinning.
Which give a very compact and small space at PCB.
2 OUTER pins for signal & 2 INNER pins for on/off ( 5 Volt )
Pin spacing is 5.08 mm
I am not sure they are of same contact data quality as PRMA, PRME.
Click the image to open in full size.
Datasheet, digikey:
http://media.digikey.com/pdf/Data%20...S4,%20SIL4.pdf
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