SX-Amp and NX-Amp

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fab

fab

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Gain

Hi Bonsai

A gain of 30db is way to high for my system since I use a preamp with gain. I usually use a gain of about 20 to 23 db for my DIY power amps ( I want to use my volume pot range a bit you know...);)
So the question is : can I reduce the gain of the nx amp and keep the stability of the amp within acceptable margin ?:rolleyes:

Thanks

Fab
 
Bonsai

Bonsai

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Bill_P said:
Appendix 1 – BOM List for 1 off nx-Amplifier Board

Item 10 lists a tab connector from DigiKey while item 12 and 14-17 list the same part number but with Mouser as a source. The part number, A100452CD-ND, is Digikey's format so the Mouser source listing is probably wrong. This is the only part that is sourced from DigiKey and if Mouser has a substitute it would allow all parts to be bought from one place.

Most resistors are listed as 0.5W metal film 1% without a distributor part number. Unfortunately resistor power ratings vary by manufacturer and I can't tell from the entry what the physical size of the resistor is. Is there a particular manufacturer and series you had in mind for the resistors? If some resistors don't really need a 0.5W rating, lower wattage parts may be easier to source or be cheaper.

I'd like to avoid using a preamp by just adding volume controls to the amplifier input. My system is presently configured that way with a 20K Ohm step attenuator and it works with my very limited shelf space. Do you see any problems doing this with the NX amp?
Click to expand...

Hello Bill

thanks for noting the one about the tab connector - I have added it to the list of updates and corrections based on everyones feedback here (17 and counting!)

Re your proposal,

1. Using a stepped attenuator is a good way to avoid 'wiper' noise and an approach I would recommend when DC coupling high input bias op-amps like the NE5532/34
2. The problem with doing it this way with the nx-Amp is that the input impedance is 10k at low frequencies, to its going to really load your stepped attenuator. Also, you are likely to get quite some roll off on HF because the attenuater resistance is in series with the input filter cap.
3. You could try dropping your attenuator to 2k (most op-amp preamps, DACs and CD players will drive 2k comfortabley) - this would give a much better result, but its still not as good as buffering your signal source first which would be my top recomendation.
 
Bonsai

Bonsai

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manso said:
Andrew in the 350 watt amp do you make use of Beta enhanced vas ??
Click to expand...

Yes, with cascoded front end, triple output stage and PSU ripple eaters.

I am getting 4ppm at 250W 20k and 7ppm at 360W (sims)

At 50W its about 500 ppb.

with straight Alexander comp, its about 13ppm at 360W 20k

I still have quite some investigation to do because I am trying some unusual comp methods (TPC, quasi TMC) and things are not as straight forward as with VFA's when you do this.


(BTW, I am getting about 70dB of PSRR at 10 Hz and around -120dB at 1kHz and it remains very good althe way out to HF (so >> 10kHz). The CFA PSRR thing is easy to fix IMV - I also suspect that this is where the real value of AFEC might be, so throw out the ripple eaters because they are big and bulky and use AFEC to get the PSRR up. When you are down at 10ppm distortion, AFEC does not add much value in terms of distortion reduction)

This project is still many months away because I am right in the middle of another big pre-amp project!
 
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Bonsai

Bonsai

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A note on the resistors re Bill_P's question:-

I sourced my resistors from Mouser. They are mostly Xircon 0.5W types wich are the standard 0.5W size - so the body is about 8-9mm long, and after you bend the leads they fit into the standard 0.5W pin spacing (IIRC that's about 12.5mm).

There were a set of resistor noise test results done by a guy from Europe about a year or two ago and posted up somewhere on DIYaudio. The Xircons were not too bad.

For the feedback resistors, I used Dale CF60 (also from Mouser) - these are very low noise, low TC devices. I read somewhere that someone did some listening tests and these were quite highly regarded. The main reason I used these is because in a CFA the feedback network resistance is low, so they can dissipate a lot of power and I did not want the possibility of any thermal distortion. On both the sx and nx-Amps, I use 5 resistors in parallel. (On my two VFA designs - e-Amp and Ovation 250 - the feedback resistor is also low, so there I used 2 half watt resistors in series for similar reasoins).

The Dales are not cheap!
 
Bonsai

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fab said:
Hi Bonsai

A gain of 30db is way to high for my system since I use a preamp with gain. I usually use a gain of about 20 to 23 db for my DIY power amps ( I want to use my volume pot range a bit you know...);)
So the question is : can I reduce the gain of the nx amp and keep the stability of the amp within acceptable margin ?:rolleyes:

Thanks

Fab
Click to expand...

Hello Fab, I have not tried it with lower gains - the loop gain plots (LTspice) look good and and the roll off is clean so I am not expecting any problems. You can experinment by increasing the value of R11 (currently 15 Ohms) to 39 Ohms - that will give you 23 dB gain

The other option is to use an attenuator on the front end - probably easier and until I am absolutely certain, a safer option.

I will add some comments into the write-up about this after I test it. Thanks for raising the point (action item #18)

rgds

Andrew
 
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I

ivanlukic

Member
Joined 2010
Bonsai said:
For the feedback resistors, I used Dale CF60 (also from Mouser) - these are very low noise, low TC devices. I read somewhere that someone did some listening tests and these were quite highly regarded. The main reason I used these is because in a CFA the feedback network resistance is low, so they can dissipate a lot of power and I did not want the possibility of any thermal distortion. On both the sx and nx-Amps, I use 5 resistors in parallel.
Click to expand...

I intend to use 1W metal oxide 2k7 resisitors - same size as 0,5W metal oxide, low inductance, high thermal stability. I guess that's possible alternative for Dales? I don't know about their noise performance.
 
P

PMI

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Joined 2013
PCB Status, Corrections

Apologies for too much cut and paste last night...
(@ Bonsai: no doubt the Merlot did not help ;))

The documentation for the nx-Amplifier is available from Bonsai's web site in a PDF document, including BOM and build instructions, here:

The Ovation nx-Amplifier V2.0

A full set of boards as shipped is shown below. Circuit boards are professionally made, double-sided, solder mask and silkscreen top and bottom side, plated through holes. They are made from Gerber plots provided by Bonsai, the OP of this thread.

nx-Amp PCB
$10 each, $20/stereo set. plus shipping and Paypal fee

PMI (2) received
pinnocchio (2) shipped/received
fab (2) shipped/received
ivanlukic (2) shipped/received
evette (4) 2 shipped/received
potepuh (2)
JethroTull (2)
Cambe (2) shipped/received
Junie (2) shipped
Bill_P (2)
mj777 (2)

nx-Amp PSU+Protection PCB
$10/board, added to the above

PMI (1) received
ivanlukic (1) shipped/received
pinnocchio (2) shipped/received
fab (2) shipped/received
evette (2) 1 shipped/received
potepuh (1)
JethroTull (2)
Cambe (2) shipped/received
Junie (1) shipped
Bill_P (1)
mj777 (1)

I expect the remaining boards to arrive approximately 8/14, and I plan to send out pm's with the exact shipping costs by that time. After that, boards will ship as payment is received. I know some of you have vacations under way or soon to start. There is no deadline ;)

Please review the BOM in the pdf document, and make sure you can source the required components, or suggested alternatives. I will try to supply anything that is easier to get in the US within reason, but this will mean that your boards may be delayed while I am waiting for parts.
 

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Bill_P

Bill_P

Member
Joined 2009
Bonsai said:
Hello Bill

thanks for noting the one about the tab connector - I have added it to the list of updates and corrections based on everyones feedback here (17 and counting!)

Re your proposal,

1. Using a stepped attenuator is a good way to avoid 'wiper' noise and an approach I would recommend when DC coupling high input bias op-amps like the NE5532/34
2. The problem with doing it this way with the nx-Amp is that the input impedance is 10k at low frequencies, to its going to really load your stepped attenuator. Also, you are likely to get quite some roll off on HF because the attenuater resistance is in series with the input filter cap.
3. You could try dropping your attenuator to 2k (most op-amp preamps, DACs and CD players will drive 2k comfortabley) - this would give a much better result, but its still not as good as buffering your signal source first which would be my top recomendation.
Click to expand...

Thanks for your thoughts on the attenuator. I was afraid I would not get by without a buffer. The question is what circuit can be used that will not degrade the performance of the system. Maybe something in your preamp project can be lifted to do this job.

I went through the entire bill of materials and put it in a spreadsheet. There were a number of part number errors that I corrected by typing them into the Mouser site as I went. The part number for D15 doesn't seem to exist. Should it be the same as for D10 and D11? For resistors I ended up choosing KOA 50ppm metal film types, even for the feedback resistors. I did find expensive non-magnetic Dale resistors but they were 100ppm so I think I will try KOA, a respected vendor. Spreadsheet is attached - it's first pass and probably needs refinement.
 

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P

PMI

Member
Joined 2013
Bill_P said:
Thanks for your thoughts on the attenuator. I was afraid I would not get by without a buffer. The question is what circuit can be used that will not degrade the performance of the system. Maybe something in your preamp project can be lifted to do this job.
Click to expand...
I would be interested in this too. If a consensus develops, and we can find something appropriate that more than one person can use, I am willing to make a layout and have a few pcb's made.

And thanks for doing the "due dilligence" on the BOM, I will look at that when I get home.

Mouser has a feature that allows you to save a BOM as a project, and then "share" the project with others via email. I have done that for the power supply boards in MrEvil's Cap Multiplier thread, and for a small number of people, it works quite well. That way one can start with a common BOM, and add or subtract parts as needed.
 
I

ivanlukic

Member
Joined 2010
Buffer should have PSU onboard and volume pot onboard so that mounting requires only one hole on the front. Volume pot should keep pcb. If small enough this pcb could fit in any power amp chassis. PSU can have have some form of voltage dropping (preregulation) onboard so that power amp transformer can be used for buffer too. I think that opamp solution would be more than OK for this purpose. Such pcb could be universal and used in almost any power amp, not only nx.

There is many j-fet discrete buffers already so I think that opamp based one would be welcome addition.
 
I

ivanlukic

Member
Joined 2010
fab said:
Hi Bonsai

The MJL series have higher HFE than the NJW one according to data sheet and are only a couple of dollars more. However, they are to-264 package instead of to-3p. Would thy fit on your actual board? Or maybe the MJW series with in-between HFE but to-247 package?

Thanks

Fab
Click to expand...

Pcb holes are big enough for MJL and TO264. I had some MJL 21193/4 at hand and they fit. I intend to buy MJL 3281/1302 for the amp (because my source does not have NJW).
 
P

PMI

Member
Joined 2013
In the case of nx-Amp, or anything similar, I think we would primarily need an active attenuator, because the input impedance is fairly low. So, unity gain, yes. I am honestly not sure if an opamp or discrete solution is better in this case, so I look forward to what other people will come up with. Good chance we can just adapt an existing circuit, or even just repackage something tried and tested here... :D
 
Bonsai

Bonsai

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fab said:
Hi Bonsai

The MJL series have higher HFE than the NJW one according to data sheet and are only a couple of dollars more. However, they are to-264 package instead of to-3p. Would thy fit on your actual board? Or maybe the MJW series with in-between HFE but to-247 package?

Thanks

Fab
Click to expand...

Fab,
I think they should fit because IIRC the pin spacing is the same.
 
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