Hi,
I am new to the amp and analogue circuit. I am currently rebuilding the A1. Recap and replace a few transistors and resistors all worked well.
Now I am annoyed by the noisy potentiometer in the line preamp section. I replaced with a new potentiometer, its still making noise when adjust the volume.
So I want to bypass the preamp entirely.
A1 has tape input which is stand alone( marked as “play” I think on the schematic attached below) there is a switch (SW1) between tape input and other inputs to separate them. So I tried to short the line preamp section, by wiring the tape input directly to the power amp input. The intention was so I when use the tape input, the amp acts as a power amp, while other inputs act as the same before.
See picture below:
tried it, didnt work well, it sounded very distorted and noisy for some reason.
my question is:
1. What could cause the distortion and noise?do I have to disconnect the line preamp section instead just short it?
2. Can I wire the potentiometer in the middle of the red wire I draw to control the volume?
here is the full schematic for reference
I am new to the amp and analogue circuit. I am currently rebuilding the A1. Recap and replace a few transistors and resistors all worked well.
Now I am annoyed by the noisy potentiometer in the line preamp section. I replaced with a new potentiometer, its still making noise when adjust the volume.
So I want to bypass the preamp entirely.
A1 has tape input which is stand alone( marked as “play” I think on the schematic attached below) there is a switch (SW1) between tape input and other inputs to separate them. So I tried to short the line preamp section, by wiring the tape input directly to the power amp input. The intention was so I when use the tape input, the amp acts as a power amp, while other inputs act as the same before.
See picture below:
tried it, didnt work well, it sounded very distorted and noisy for some reason.
my question is:
1. What could cause the distortion and noise?do I have to disconnect the line preamp section instead just short it?
2. Can I wire the potentiometer in the middle of the red wire I draw to control the volume?
here is the full schematic for reference
The noise on the potentiometer is a result of a DC offset between the inverting input and output of IC1 causing wiper noise . Replacing the IC with a suitable low offset device may fix the issue.
Alternatively replace this stage with a conventional design: MF A1 modifications
Alternatively replace this stage with a conventional design: MF A1 modifications
I tried to rewire it to the right hand side of R4 as you suggested in your picture, worked nicely right away.
the other inputs worked just as before. So it worked as what I wanted it to be.
I didnt have a 12k resistor in my hand at the moment so didnt replace R4. Do I really need to increase the resistance of R4? How does it affect the original line preamp circuit if R4 value increased?
To add a capacitor in series with R34, I have 1uf film capacitor, will it work instead of 10uf?
It will work with two in parralel to match the feed pairs C6 & C7.
You do not need film capacitors, electrolytics are fine.
I actually found that direct feed from my DAC to power amp section of A1 provides enough gain for me already. The volume is now controlled using software on my PC, which I know is not optimal as software volume control brings in losses.
so I am wondering if I can just wire the potentiometer in to control the volume?
Its basically the same as what I just did except R4 is not removed yet. it seems leave the preamp circuit there doesn’t affect it.
My problem now is the volume control, so I am thinking to wire the volume controller in.
Your suggestion actually reminder me another of my issue I mention in another thread after my recap work: the crackling noise issue on my right channel.
I was suspecting the noise came from cool soldering. But now I suspect its from the degraded input selector switch.
so I removed R4 just now, and problem solved! Absolutely no noise on either of my channel, perfect.
there are so many things can go wrong in this amp, input selector, potentiometer, so I think the best way is to just use it as an power amp.
here is the amp currently running as power amp only feed from the pre-out of my NAD c390dd (act as dac+preamp)
I know its getting hot quickly with lid off, so only run for 5 mins in this test mode
Interesting, I have exactly the same problem now. Mine one was recapped two years ago, running warm for two winters time.
The selector is getting noisy last two weeks and disconnect sometime, I need to "touch" or switch it around to get the sound back. With my The DIY Nutube Preamp - FIRST WATT ready, I really want to know how to "skip" the noisy pre-amp with minimal modification!
Any response will be appreciated! Thanks.
The selector is getting noisy last two weeks and disconnect sometime, I need to "touch" or switch it around to get the sound back. With my The DIY Nutube Preamp - FIRST WATT ready, I really want to know how to "skip" the noisy pre-amp with minimal modification!
Any response will be appreciated! Thanks.