Really simple preamp, could someone check it ? - Page 2

tiroth · 10th September 2003, 06:27 PM

It may not seem like much but the LED and bleeder resistors are wasting valuable battery life. For SLA I wouldn't bother removing them, but those 9V get expensive fast.

00940 · 10th September 2003, 07:11 PM

Well, I've no problem removing the led, it was just cool.

The choice between a voltage divider and the ps below is more tricky. If a battery went to fail, we'd have a severe problem of voltage balance. I know this way is used in the grado ra-1 but most diyers on headfi or headwize would use a setup like in the two posts above. Is it too much of a fear ?

jean-paul · 10th September 2003, 07:59 PM

The input caps should be 1 uF instead of 0.1 uF.

carlmart · 10th September 2003, 10:57 PM

Quote:

Originally posted by 00940
Well, I've no problem removing the led, it was just cool.

The choice between a voltage divider and the ps below is more tricky. If a battery went to fail, we'd have a severe problem of voltage balance. I know this way is used in the grado ra-1 but most diyers on headfi or headwize would use a setup like in the two posts above. Is it too much of a fear ?

Yes, using a continuously lit LED with 9v batteries will drain them. You may use blinking types.

If you use batteries you may also think on some way to know when they get below a certain voltage. Most modern ICs work down to +/-4.5 or less though, but you will lose some dynamics.

A voltage divider is certainly a good option, but it's far from simple. Now that you put capacitors at the input and output you may go back to it.

The chance of a battery failing is almost as likely as an AC supply failing. A battery should go down slowly, and both at the same time. What you should never do is not putting fresh same type batteries for BOTH units when you change them. Same type and same charge.

In any case DC is a seriosu problem in two cases:

1) If you send DC to an MM or MC cartridge

2) If you send DC to a power amp.

That doesn't mean you should noe prevent that.

Personally I think you should only use blocking capacitors at the power amp input and use low offset chips all along the preamp chain.

Carlos

Werner · 11th September 2003, 05:39 AM

Quote:

Originally posted by jean-paul
The input caps should be 1 uF instead of 0.1 uF.

The input caps should go, as we trust it
that the sources are reasonably free of DC.

Please use JFET-input opamps that are known stable at unity gain.

With a better-controlled power supply you could drop the output caps, but I guess that with batteries this could become a risky business.

jean-paul · 11th September 2003, 08:20 AM

Quote:

The input caps should go, as we trust it
that the sources are reasonably free of DC

It can be dangerous to trust unknown things.

Werner · 11th September 2003, 08:33 AM

Sure. But the source is known. Either a commercial source with belts-and-braces protection (made in Japan-style), or else a DIY source that is definitely known and where it is your very own responsibility to keep the output free of DC.

And anyway, given the presence of the output coupling caps, the input ones do nothing useful once JFET opamps or opamps with nulled biased current are used.

MBK · 11th September 2003, 09:30 AM

Forgive me, but the 0.1 uF caps for the PSU I hope do go to ground and not in series with DC...?

cdl · 11th September 2003, 01:36 PM

00940, you might want to consider putting a little bit of gain in to compensate for loss in the pot (depends on the sensitivity of your power amp).

Furthermore, I would heavily recommend getting a couple of opa627's as free samples from TI - they are very nice sounding, and expensive... but who cares when you get them for free?
Separating the two channels in distinct chips should give better channel separation.

I don't really think you need those coupling caps - as long as you are careful with changing the batteries, and remember to monitor for DC every time you change something. Newer sources do not output DC, but measure for yourself!

I have the prototype shown here playing, and it is pleasant (with opa627 instead of op37). You might not want the "cheap" class A biasing 15k resistor from output to -, though... and you can make do with 100 ohm output resistor instead of 470.

Your RF input filter is probably a good idea - do keep it.

Despite a small +/- voltage imbalance in the power supply that I haven't got around to fixing yet, DC levels are below 0,1 mV measured with my DMM. My power amp doesn't have a problem with that at all (although it is DC coupled, yes!).

cheers - cdl

00940 · 12th September 2003, 05:30 PM

I forgot those samples

well, it's repaired, I've a pair of op627ap and a pair of buf634p on their way from TI, no questions asked. I also took a pair of lm317/337 voltage regulators.

With those pieces saved, I could try something slightly better. I'm planning to use the zero distortion power supply. No more problems about batteries. For the rest, I could go for this (pcb perhaps still available from macura) :

Thanks a lot to all of you, your comments helped me a lot.

10th September 2003, 07:59 PM	#13
jean-paul diyAudio Moderator Emeritus Join Date: Sep 2002 Location: Germany	The input caps should be 1 uF instead of 0.1 uF. __________________ It's only audio

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Intrest Check : Pretty Simple Digital Preamp / Volume Control	rhysh	Chip Amps	69	20th December 2008 06:14 PM
Check: Simple Circuits	Devil_H@ck	Parts	19	9th June 2004 02:54 PM
check my simple circuit	svokke	Solid State	5	25th March 2004 06:19 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

10th September 2003, 06:27 PM	#11
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	It may not seem like much but the LED and bleeder resistors are wasting valuable battery life. For SLA I wouldn't bother removing them, but those 9V get expensive fast.

11th September 2003, 08:33 AM	#17
Werner diyAudio Member Join Date: Jan 2002 Location: Belgium	Sure. But the source is known. Either a commercial source with belts-and-braces protection (made in Japan-style), or else a DIY source that is definitely known and where it is your very own responsibility to keep the output free of DC. And anyway, given the presence of the output coupling caps, the input ones do nothing useful once JFET opamps or opamps with nulled biased current are used.

11th September 2003, 09:30 AM	#18
MBK diyAudio Member Join Date: Apr 2002 Location: Singapore	Forgive me, but the 0.1 uF caps for the PSU I hope do go to ground and not in series with DC...?

11th September 2003, 01:36 PM	#19
cdl diyAudio Member Join Date: Aug 2003 Location: Copenhagen	00940, you might want to consider putting a little bit of gain in to compensate for loss in the pot (depends on the sensitivity of your power amp). Furthermore, I would heavily recommend getting a couple of opa627's as free samples from TI - they are very nice sounding, and expensive... but who cares when you get them for free? Separating the two channels in distinct chips should give better channel separation. I don't really think you need those coupling caps - as long as you are careful with changing the batteries, and remember to monitor for DC every time you change something. Newer sources do not output DC, but measure for yourself! I have the prototype shown here playing, and it is pleasant (with opa627 instead of op37). You might not want the "cheap" class A biasing 15k resistor from output to -, though... and you can make do with 100 ohm output resistor instead of 470. Your RF input filter is probably a good idea - do keep it. Despite a small +/- voltage imbalance in the power supply that I haven't got around to fixing yet, DC levels are below 0,1 mV measured with my DMM. My power amp doesn't have a problem with that at all (although it is DC coupled, yes!). cheers - cdl

12th September 2003, 05:30 PM	#20
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	I forgot those samples well, it's repaired, I've a pair of op627ap and a pair of buf634p on their way from TI, no questions asked. I also took a pair of lm317/337 voltage regulators. With those pieces saved, I could try something slightly better. I'm planning to use the zero distortion power supply. No more problems about batteries. For the rest, I could go for this (pcb perhaps still available from macura) : Thanks a lot to all of you, your comments helped me a lot.