Increasing The Gain on a Pre-amp

In answer to your question, it is probably is loud enough it probably me having a bad day and getting an idea in my head that until I try a few things out it wont go away

Know how you feel, it's when you spend thousands on test equipment and then find out hardly any of it makes a real difference. Still nice to know first hand, and fun.

[sp10mk11]

Hello All
I have ordered the book suggested by BikerHiFiNut and having had a word with one of the gods on here I am going to put my serious reading head on (no more Viz annuals) and actually try and learn something and keep it in my head.

Just out of interest I was advised by Restoration73 try dropping slightly the value of Resistor R622L from 8.2k, I dopped to 5.6k and indeed this does raise the sound level, it is almost spot on actually, unfortunately the house is busy at present so I cannot do a full test.

I haven't found the velleman pre-amplifier so I ordered another one and will have a go, I am sure this will work but it will need refining I am sure.

The book is on its way when it arrives it will be read in my download chamber its the only place I can concentrate

[bikerhifinut]

Its an easy book to read Gary, the tome suggested earlier up will tell you more for sure. But i always feel that if you arent bogged down in technical stuff and theory and are given the stuff you need, you are more likely to have a crack at creating something. You can improve your knowledge once your curiosity is sharpened i think.
But you'll get enough out of it to be able to tweak and adjust that little amp. The book has a few good circuits to work with too.

Andy.

[sp10mk11]

Thanks Andy
I am looking forward to seeing this book, providing I can concentrate I will be ok, see this comes with old age.
Thanks again
Gary

[sp10mk11]

Got the pre-amp today got the book! errr? "Super Student Grub" grrr the idiots have sent the wrong book

oh well half way there


Gary

[Radio Wrangler]

Don't underestimate the dangers of student grub. It can be as dangerous as working on power supplies. I work on a university campus and early every year as students discover that their funds won't stretch to the on-campus eateries for everything, there is a surge of fire brigade callouts as they learn to cook. All the smoke alarms are connected to fire control panels linked to automatic calls to the brigade. The electronics works reliably - it gets 'tested' often enough.

David

[sp10mk11]

ha ha yes I can well believe what you say David, I resisted looking inside, though the picture on the front looked tempting on its own!

So much for the company "Greener Books" I now have to drive into saltash (3 miles) to the Post Office to send this back, ha ha we make sport for one another

[sp10mk11]

SSHHHH I am reading

Ps it all works and has been fun building and setting it up

[Radio Wrangler]

When you've finished reading, there'll be a small test.

David

[bikerhifinut]

Originally Posted by sp10mk11

Ps it all works and has been fun building and setting it up

All very encouraging Gary.
I was intending to get my Velleman kits out and build them today but In a moment of madness I decided to upgrade my laptop to W10. nuff said.
I will get round to it this week and I'll compare notes with you if you like?
I think it could make the basis of a sticky for the frequent queries that come up when people want to boost a signal, say for a Gramophone that used a high output Xtal and all you can get is a med or low output Ceramic cart for instance.
I'm going to also build the Phono preamp too and have a listen on the Hi fi, if it passes muster it might be a useful wheeze for the lads with better quality gramophones equipped with SP25 or MP60/P128 etc.
Catch you in the messages.

Andy.

[sp10mk11]

Some figures.
Basically I fed a test tone into the tuner input
this measured 230mv 400Hz,
Input board to pre-amplifier 115mv, This section has all the input selectors and phono stage on it.
After added op amp stage 535mv into the volume control
After some fiddling with pre-sets in place of the 1 meg supplied feedback resistors I found 47k feedback resistors gave the amount of boost I required
Gary

[bikerhifinut]

Good stuff.
You know you can calculate the feedback values without having to mess about with presets?
Bearing in mind certain combinations can affect the noise performance of the op amp and stability.
At line levels It's probably academic though. I might have just left the presets in the feedback loop as a handy way to adjust the gain over a limited range.
And has it affected the sound quality or as expected/hoped is it neutral in its effects?
A.

[sp10mk11]

Hello Andy
I for got to mention that I replaced the 1uf electrolytics with 2.2 uf Polypropylene ones and also removed 2 of the 4 output caps, two from the op amp to the pre-sets for the output level, and then two from those to the input, So removing the pre-sets allowed me to lose a pair of caps

What I can say is that I cannot notice any detriment to the sound at all, apart from the difference in volume at a given volume setting it is Ticketty Boo

Gary

Attached a picture of the circuit board mounted at convenient spot on the amplifier board, it is run off a 26v supply via a 15v LM voltage regulator

That is something I did not know (amongst a million things) that you can calculate the values of the feedback resistors for a given gain

[Radio Wrangler]

That looks like a success. Opamps are useful little beggars and you can teach them to do all sorts of tricks.

David

What I can say is that I cannot notice any detriment to the sound at all

Easy with good engineering practice, as you have found.

[sp10mk11]

Hello Everyone
I am still gobsmacked at I have done thanks to the members on this site.
My mind is racing around thinking of pre-amps etc that can built now.
Fear is a strong limiting factor for me, I do not handle disappointment well, but the kit and guidance from here as shown me what is possible with this little device.
When I think of all the jobs I wanted to do in the past or have done but not understood what I was doing, I just built them as instructed and of course they worked but I was always nervous of them.
PHEWWWW

Well I am still reading my little book, there is a whole world out there me thinks.

Now I read that Op-amps vary performance amongst the same type can anyone throw any light on that, or throw it out of the window if what I have said is a load of rubbish.

Big Thanks Again to all who have contributed to this learning curve

Gary

Throw it out of the window! Any modern op amp will work very well.

[bikerhifinut]

At line level, most Op amps from the TL072 onward will likely be fine and dandy in most domestic Audio applications.
As they are cheap enough to buy, if you felt so inclined to prove to yourself, you could buy a small selection of Audio Op Amp Chips and substitute them in the 8 pin DIL socket. It's very doubtful you'd notice any difference especially in that particular circuit. I suspect the resistors in the circuit would contribute more noise.
Theoretically a 5532 and then something like an OPA2604 would be better, but I suspect it all depends on the circuit as there are other ways to use an Op amp.
Where Op amp quality does become critical would be at very low signal applications, for example a moving coil cartridge step up amplifier/first stage. Primarily due to noise performance.
Some professional audio designers use discrete transistors in areas like this for that reason, I am led to believe.
It's an area that will of course arouse strong feelings on both sides of the coin.
Just a few things to chuck into the mix.

A.