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Old 22nd August 2013, 08:18 AM   #21
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The input cap is optional for sources that have DC offset. Output cap is a must. Not a problem if the power amp is DC coupled. It is better to have some point where DC loop is broken by a cap. More then once I have seen servo's of DC coupled pre and DC coupled power amp interacting...

Quote:
Originally Posted by Bonsai View Post
My post should read '100 Ohm and 47pF'
Please make sure to use film caps and not ceramic here. 100 pF styroflex/MKP/PPS (in SMD !) are standard values so I would pick those. You made a very good point regarding the filter before the potentiometer !
I think filtering should be done at a lower frequency otherwise it makes not much sense. 1 kOhm and 100 pF ? My calculator is gone and I am fighting with the calculator in Windows 7 so I am taking a guess here.
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Last edited by jean-paul; 22nd August 2013 at 08:31 AM.
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Old 22nd August 2013, 09:50 AM   #22
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We should retain output cap, but we could probably use two electrolitics in series with negative poles connected to form a bipolar, footprint would be small and filtration very good (my Marantz CD57 mk2 uses two Elna 220uF/16V caps in series at the output). 10uF film cap will be huge, hard to obtain and the pcb will have to "grow".
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Old 22nd August 2013, 12:08 PM   #23
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Quote:
Originally Posted by jean-paul View Post

I think filtering should be done at a lower frequency otherwise it makes not much sense. 1 kOhm and 100 pF ? My calculator is gone and I am fighting with the calculator in Windows 7 so I am taking a guess here.
I would be reluctant to add too high resistor value in series with the input of j-fet that is already at some disadvantage in comparison with bjt when it comes to noise performance. I think that's the reason why Bonsai proposed 100R. In case that we want lower frequency for the low pass filter we could use more capacitance, say 100-220pF polystyrene or polypropilene (footprint will be the same). But, that's for each builder to decide what to use.

For the first version of the pcb I would suggest TH components, j-fet and bjts that is easy to obtain from mainstream distributors. We could always make revision of the pcb that will use SMD. We must consult PMI who will make pcb layout what is his choice.
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Old 22nd August 2013, 02:14 PM   #24
Bonsai is offline Bonsai  Taiwan
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I've kept te input filter fo high because most power amps will already have an input bWL filter. The main purpose of the filter is just to roll it off at HF.

As pointed out, the PCB should be flexible enough so the builder can adapt for their specific situation.
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Old 23rd August 2013, 12:10 AM   #25
Bonsai is offline Bonsai  Taiwan
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How about

SMD for all bipolar devices (SOT 23)

TO 92 for the JFET

SMD for the zeners

0.5W metal films for the resistors ( I have a good selection in my parts cupboard!)

Standard TH for caps

For the power dropper resistors feeding the Zener regs, 4W ww types - commonly available and cheap.

Any views?
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Old 23rd August 2013, 04:44 AM   #26
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Quote:
Originally Posted by Bonsai View Post
How about

SMD for all bipolar devices (SOT 23)

TO 92 for the JFET

SMD for the zeners

0.5W metal films for the resistors ( I have a good selection in my parts cupboard!)

Standard TH for caps

For the power dropper resistors feeding the Zener regs, 4W ww types - commonly available and cheap.

Any views?
Footprint for big wire wound resistors should allow ordinary horizontal mount resisitors to be mounted vertically to save pcb space. Vertical mount types are not easy to find.
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Old 23rd August 2013, 06:55 AM   #27
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4 Watt wire wounds ??? This was supposed to be small and we wouldn't use tube stuff isn't it ?

Separate regs for each channel fulfill same purpose but make better power too. Can be heard at first spot. Simple discrete regs that can handle 50 V input voltage or more are an option too. What is the purpose of a buffer with mediocre power supply feeding an excellent power amp ? Weakest link is weakest link.

BTW Alps RK27 is not very good. On Ebay one can buy DACT 21 stepped attenuators that are way better and cost less. Pads for RK27 allow for DACT 21 to be used.

Better first define design goals more precise...I have more than once made stuff because it needed to be either cheap, too simple or very small that did not fit the bill and were left unused after testing. When DIYing one should go for the best otherwise it has no sense. DIY amp will be more expensive than ready-made anyway. Bonsai, your power amps really deserve good buffer amps.

Maybe design goals should include quality (more on the foreground than size or having certain parts).


Quote:
Originally Posted by ivanlukic View Post
We should retain output cap, but we could probably use two electrolitics in series with negative poles connected to form a bipolar, footprint would be small and filtration very good (my Marantz CD57 mk2 uses two Elna 220uF/16V caps in series at the output). 10uF film cap will be huge, hard to obtain and the pcb will have to "grow".
Wima MKS2-XL exists in 10 ”F. 5 mm pitch and size almost equal to electrolytic caps.... Sounds good, no need for series arrangement or replacement after X time. I have also tried the Vishay 10 ”F 63 V as used in VSSA amps. Those are better but they are a tad larger (but not very large). Choosing electrolytic caps in this case is a design error IMO. I will be happy to send you 2 x large value small size film caps for your marantz. I think you will be surprised. This was one of the mods I did a lot in the past with marantz stuff.
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Last edited by jean-paul; 23rd August 2013 at 07:17 AM.
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Old 23rd August 2013, 09:33 AM   #28
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If we want this pcb to be appealing to most potential group buyers it should be practical. It is not supposed to be uncompromising in quality. We see that even the most basic circuit (without cascoding or bootstraping) has distortion 0,000...% and decent drive capability. With separate RC filtering for each psu rail for each channel we should have good noise performance too. I think that is good enough for most diyers.

Practicality criteria are:

1. Cheap (yes, Wima MKS2 is small but it cost 4.- Euro each. Apart from Alps RK27 pot that's more than all semis in the circuit. Is it practical to use such part?)

2. All parts TH. Most builders are not excited about using SMD parts.

3. All parts available from one mainstream distributor (DACT 21 is realtively cheap good quality part, but separate e-bay order just for the pot + all expenses, waiting, etc... not practical. I do not have Pay Pal account so for me that is not option)



PS

It seems that DACT 21 has the same footprint as Alps RK16, which is different from Alps RK27. But using DACT 21 means that we loose important RK 27 property, which is to hold the whole pcb. With DACT 21 we would have to run some kind of wires from the pot to the pcb.

I know how warm is LED resistor in my preamp that drops only 18V. In this circuit we have to drop 30V or more. It's nice to have at least 2W dropping resistor.
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Old 23rd August 2013, 10:23 AM   #29
Calvin is offline Calvin  Germany
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Hi,

I suggest to design for a more common supply voltage range of +-12V to +-15V for TH and +-5V to +-9V for SMD.
Especially SMD would run into heatpower dissipation problems as useable JFETs only come in SOT23 casings allowing not much more than 100mW reasonably.
Anybody using higher rails would be free to add a dropper stage before the circuit anyway.
IŽd also suggest to not include the Poti, but to design jumpers instead.
This would leave the user the choice of using any Poti he likes, or to omit with a Poti alltogether.
If one designs a SMD layout it should preferrably be SMD throughout and not mixed up SMD/TH.
The Poti value may be chosen higher than 10k, say 50k.
This would rise the input impedance from <10k to ~33k, thereby loading the source less.
Noisewise thereŽs no significant loss in noise figure here with higher Poti ohmage.
The input filter as I suggested in #12 filters only >2MHz.
I wouldnŽt raise the bandwidth further to keep HF safely out of the circuit.

jauu
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Old 23rd August 2013, 10:48 AM   #30
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Good ideas. I would also place Poti and buffers + integrated PS at the back of the case in the vicinity of input circuitry and use an axle extension rod. An extra mini toroid (Nuvotem: 10 to 15 Euro) won't break the bank. Why waste energy and convert it in useless heat by feeding the circuit high + and - voltages ? I already see the heat coloring the PCB ....Dropper resistors mean that DC wiring has to made to the front panel which is not practical. Also reliability is in question.

"good enough" is not good enough in DIY audio A film cap has so much advantages that I would not even think of using electrolytic caps in the signal path. I buy the Wimas in the hundreds and then prices go down. If it must be cheap then it has no sense to build it. Prices of parts are decisive in ready made gear and we all know what that leads too.

Sorry but I am out of this thread as I feel I create too much unwanted noise. IMO DCB1 Mezmerize is a more appealing choice as it is also class A, has shunt PS, input relay switching, muting relay and volume control on one single beautiful and relatively small board. Besides that it is not expensive to build. Having built at least 4 of them I can tell it sounds very very good. A practical buffer with many win-win points.
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Last edited by jean-paul; 23rd August 2013 at 11:08 AM.
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