DIY Transistor Amp Using NVA Boards

For the three and more legged things
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#151 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

kptseng wrote: Thu Aug 27, 2020 11:07 am
Nick wrote: Sun Aug 23, 2020 6:10 pm If you are using choke input make sure you are pulling enough current for your choke inductance.
Hi Nick,
I read about the need to have enough current for the LC filter to work, but I’m not sure what I need to do to ensure that.

My plan is to have 30H/10Amp choke, followed by 2x10,000uF for each rail. The transformer size will be 250VA with 2x35vac secondary for the +ve and -ve rail. Would that be ok?

Also, would adding a soft start module before the transformer cause any issue (to the current pull)?

Thanks
Well, its its 10H (which as ed points out, will be huge) and not 30mH then anything moire than a couple of ma will be fine, so a 15k resistor across the output will mean its remains in choke input. You will get about 31v out of the supply. Not sure why you need a 10A choke to supply the driver stage though.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
kptseng
User
Posts: 11
Joined: Mon Jun 15, 2020 1:06 pm

#152 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

ed wrote: Thu Aug 27, 2020 12:11 pm
kptseng wrote: Thu Aug 27, 2020 11:07 am

My plan is to have 30H/10Amp choke,
??? wow, how big is it?
Opppsssss sorry, I got it wrong. It’s only 300mH 1A....... :-(
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#153 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Pk, so 300mH 1A makes more sense, so then you need to draw mor than 100ma at all times. So 240R 10W resistor would do the job. But that will get hot as its dissipating 4W or so.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
kptseng
User
Posts: 11
Joined: Mon Jun 15, 2020 1:06 pm

#154 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Nick wrote: Thu Aug 27, 2020 2:12 pm Pk, so 300mH 1A makes more sense, so then you need to draw mor than 100ma at all times. So 240R 10W resistor would do the job. But that will get hot as its dissipating 4W or so.
Thank you Nick, but where do I place the 240R resistor? After the L and before the C?
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#155 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Across the C, so it provides a guaranteed load for the supply to keep it in choke input. Myself I would not bother and use a regulator, or just use the amps as they were.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
kptseng
User
Posts: 11
Joined: Mon Jun 15, 2020 1:06 pm

#156 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Nick wrote: Thu Aug 27, 2020 4:49 pm Across the C, so it provides a guaranteed load for the supply to keep it in choke input. Myself I would not bother and use a regulator, or just use the amps as they were.
Thanks Nick, but pardon my ignorance, is the resistor parallel to ground at C or is it in series from C to output?
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#157 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

kptseng wrote: Fri Aug 28, 2020 5:40 am
Nick wrote: Thu Aug 27, 2020 4:49 pm Across the C, so it provides a guaranteed load for the supply to keep it in choke input. Myself I would not bother and use a regulator, or just use the amps as they were.
Thanks Nick, but pardon my ignorance, is the resistor parallel to ground at C or is it in series from C to output?
Across the C so from the voltage output to 0v,
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
kptseng
User
Posts: 11
Joined: Mon Jun 15, 2020 1:06 pm

#158 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Thank you Nick, learn something new again.

You mentioned about drawing 100mA, but for driving the output stage would it be too low?
If I am not wrong NVA's quiescent current is approx 500~700mA.

Thanks.
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#159 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

You need to understand choke input power supplies.

If this is for the output stage it may be fine without the bleed resistor if the output stage draws 100ma. Choke input supplies have a minimum current that must be drawn for them to stay into choke input. If they leave chole input the output voltage changes from 0.9 times the input voltage to 1.4 times the input voltage. This may or may not cause a problem.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
kptseng
User
Posts: 11
Joined: Mon Jun 15, 2020 1:06 pm

#160 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Hi Nick, thank you so much!!

I think I’ve got a much better understanding than before.

Saw this discussion on DIYaudio as well https://www.diyaudio.com/forums/power-s ... upply.html

Cheers
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#161 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Deleted.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#162 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Well I deleted the last post as the writing didn't match up with the photos. I was one out all the way to the bottom, so let's hope I've got it right this time. :roll:
Right!
I get bored without something to tinker with, so the other day I dug out my transistor power amp, with the boards generously donated by the late Richard Dunn. So I guess we had a strange hybrid of his design mixed in with my attempts to make a decent power supply.
This amp almost made me give up on valves, but for one bugbear. It blasted the room out on my 98dB speakers on nothing more than a slight turn of the volume pot on the NVA passive preamp I had it connected to. In practical terms, in my tiny living room with those big, efficient speakers and despite its good sound, it was unliveable with long term because basically it was a poor match with my speakers.

Anyway I thought I'd give it another go, and with the previous mismatch to my speakers at the forefront of my thinking, I turned it into an integrated amp with 3 inputs. I used an attenuating potential divider of 15K top and 10K bottom resistor to drop the 2V RMS digital input levels to 800mV before they reached the volume pot. I know this is not regarded as an ideal solution to excessive input levels, but beggars can't be choosers, and it has made the amp usable. 9 0'clock on the volume control gives decent sound levels and it will kick butt at 11 o'clock, with neighbour annoying levels occurring at 2 o'clock. That's a far better usable range.

Selector switch is on the back panel. The volume pot is close to the rear and is actuated by a coupling rod which connects to the knob on the front panel.
IMG_0857.jpeg
IMG_0857.jpeg (78.38 KiB) Viewed 2382 times
It sounds clean neutral, liquid and plays the music with unflustered authority no matter how busy things get.

I have a broken Phono1 in the loft too. Nothing major, just one of the legs on the reg chip on the negative side of the regulator board was broken (I did it) and I couldn't be arsed to fix it, so bought our Ant's MoFi phono stage. The reason for me not being arsed, was that it picked up mobile phone signals, which was bloody annoying to be honest. I suspect a lack of RF screening caused by the acrylic enclosure. The MoFi suffered none of these issues and sounded fine with my MM carts so that was that.

Anyway looking for something else to tinker with, I ripped out the boards from the Phono1, fitted a new negative regulator chip and mounted the phono boards inside the integrated amp chassis, powering the regulator from the input stage power supply. It runs at plus and minus 22V, which is the recommendation for the op-amps it uses. No RF pickup now either.:
IMG_0859.jpeg
IMG_0859.jpeg (120.42 KiB) Viewed 2382 times

Close view shows the phono boards mounted on a stiff plastic plate, which is in turn epoxied to an aluminium bar that runs front to rear inside the chassis
So now, the record player can simply be plugged into the back of the amp, eliminating a phono cable and four sockets from the equation
IMG_0859 (1).jpeg
IMG_0859 (1).jpeg (138.16 KiB) Viewed 2382 times
The sound of this amp is giving me a troubling set of choices to make. If the 45 amp is to compete with it for resolution and clarity, on anything other than girls and acoustic guitars, then it is going to require far better output transformers than the ones I converted from push-pull to SE operation. But then again; back in June last year at Steve's place, Nick Paul and Steve himself did try to tell me about that issue.
I didn't really understand what they meant, but it's now painfully evident what they were talking about.
Gonna have to save up my pennies by the looks of things.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#163 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Why does the 45 have to compete? There is no award for the winner.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#164 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Nick wrote: Wed Jan 05, 2022 6:26 pm Why does the 45 have to compete? There is no award for the winner.
Guess you’re right Nick. They are both perfectly good amps and both can be enjoyed.
Last edited by Cressy Snr on Wed Jan 05, 2022 6:48 pm, edited 1 time in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Ant
Shed dweller
Posts: 2332
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#165 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

You can borrow the james transformers i have on eds sv811 to try if you want
They are 5k iirc, not sure if they have the taps on them for different loads though.
I will need them back at some point when i finally get round to putting it in a box
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Post Reply