One more buffer question...

[Adam M.]

Can a buffer be powered from the main gainclone power supply, using a voltage divider and resistors of high enough resistance to supply just enough current to the buffer as to not waste power?
THanks,
Adam

 

[joensd]

If you´re talking about an opamp-buffer:
you could use zener diodes to reduce voltage for the buffer or something like 78L15/79L15.
If your voltage rails are <24V use OPA604 for example.

 

[Adam M.]

Any suggestions on zener diodes? I am thinking about using an OPA627 whenever I get around to build it... I'm thinking abuot 12v @ 250ma should be enough, does that sound reasonable?

Are there any sonic benefits to using a voltage regulators? My input voltages are going to be right arounf 35V (the transformer is the only part I have in my hands as of yet). The Zeners are so cheap ...3 cents a piece from digikey for 12v 500ma, and that makes me nervous.

THanks,
Adam

 

[joensd]

Hi Adam,
do you mean your voltage rails are +-35V or just 35V?
How did you come to a 12V@250mA (again +-rails?) ?

If you really need that "much" current there´s no question you shouldn´t use zeners to regulate due to power dissipation and bad regulation.
Actually even the 7815/7915-regulators for example (1A-version) are dirt cheap (in comparison with the OPA627 especially)and give better regulation.
And using such a brilliant opamp you´ll probably want the best regulation anyway.

Give some more details of your circuit idea; that´d help.

Regards
Jens

PS.: you could also think about using something like a BUF634 (DIP8 or TO220 for greater power dissipation) which is a brilliant sounding buffer that is a bit cheaper than the OPA627)

 

[joensd]

One question to you:
Have you thought about a discrete buffer like a source follower?
Probably the purest, cheapest and easiest solution except it definitely needs a regulated supply.

 

[Adam M.]

The 12v @250mA I came up with from about 30 seconds of thinking after seeing what input voltages the opa627 needed and from remembering back to my op amps class... for some reason I remember feeding a chip that much current and didn't want to be current-limited, I have no basis other than that for the current need.

I did mean +-35v on the power....

My current plan (which changes daily it seems as I learn more and more about practical implementations of these circuits) is to build a PA100 directly from an-1192 with a buffered input and a stepped attenuator of some flavor in front of the buffer since I don't have a preamp yet.

The reason I am thinking PA100 is because I will use this amp on occasion to drive both 4 and 8 ohm speakers in a room with a 15' high cathedral ceiling, I want to have enough power on tap.

Thanks for the suggestion on the BUF634, I"ll read up on it (and I'm sure ask 100 questions)

What is a line source follower?

Thanks for the help, hope I gave you the information you needed.
-Adam

 

[Steve Eddy]

Sorry if I might have missed something along the way, but what exactly is the point of adding a buffer to the input of a Gainclone?

se

 

[Adam M.]

A few threads back I asked how to control the volume on a PA100 amp and someone suggested putting an attenuator in front of a buffer ...So I tried to learn as much about implementing a buffer on a gainclone in as short of a time as I could... And here I am. If my quest is misguided, let me know. I also wanted to push myself a little from what I did in labs in school, so its a bit of a learning experience here and a bit of ignorance on how these amps need to be set up in a practical application, where the input source isn't a perfect square, sine, or triangle wave coming from a machine designed to send signals into such chips and the supply voltage isn't dialed in on the machine next to the signal generator.
I'm all for learning, please teach.

 

[Steve Eddy]

A few threads back I asked how to control the volume on a PA100 amp and someone suggested putting an attenuator in front of a buffer ...So I tried to learn as much about implementing a buffer on a gainclone in as short of a time as I could... And here I am. If my quest is misguided, let me know. I also wanted to push myself a little from what I did in labs in school, so its a bit of a learning experience here and a bit of ignorance on how these amps need to be set up in a practical application, where the input source isn't a perfect square, sine, or triangle wave coming from a machine designed to send signals into such chips and the supply voltage isn't dialed in on the machine next to the signal generator.
I'm all for learning, please teach.

Well unless you're doing an inverting version of the PA100, can't think of any particular reason why a buffer would be needed. But if you want to add one just as a learning experience, it can't hurt.

se

 

[peranders]

Sorry if I might have missed something along the way, but what exactly is the point of adding a buffer to the input of a Gainclone?

se

I have this option in my Gainclone and the point is:

1 Lowering the offset (possibility to this)

2 Low source impedance for the LM3886 (positive thing)

3 Possibility tayloring the frequency characteristics, using small (not huge) coupling caps.

4 Creating high input impedance which sometimes can be a positive thing.

When I read posts about LM3875 and LM3886, many troubles comes from problems with offset and overall bad design around the passive parts. With a buffer you can get a more predictable and equal result.

 

[Adam M.]

Thanks for the responses, Think I am going to use the buffer just for the sake of using the buffer, and with the suggestion of a BUF634 instead of an OPA627, it just got a lot more economical.
The high wattage resistor/voltage divider idea is still floating around. Is there any electrical reason not to do it, I'd like not to fry the buffer when I get around to building this thing. I know it may not be optimal... but are the adverse affects any worse than a bad sounding amp?

 

[millwood]

The high wattage resistor/voltage divider idea is still floating around. Is there any electrical reason not to do it,

if I read you correctly, you wanted to use resistors in serial to the rails to reduce voltage. The thing is that 1) the supply voltage on your (IC) buffer then depends on load current. When there is no load, there would be no voltage drop off those "serial" resistors and your IC buffer is being supplied by +-35v rails. and 2) you are increasing the power supply's internal resistance. so when the output voltage goes way up, the supply voltage goes way down - not something you want to do.

I would use a pair of zeners depending on the IC's supply voltage (12v?) with decoupling caps.

You can use also a follower and then directly hook it up to the 35v rail. I am running a very simple one right now with great success.

 

[Adam M.]

For the voltage divider, I was thinking 2 resistors in parallel to the rails actually, calculating the resistors so there is the proper voltage between the 2 resistors... for example.. (using non-standard resistor values because I'm too lazy to look up standard values...

35V rail------------
|
325ohm
|
+--------12 V comes out here @ ~70mA.
|
175 ohm
|
0V------------------

I'll have to read more about whatever buffer I settle on to set the proper current.
Only problem I see is that the resistors would have to be fairly high power, in the neighborhood of 3 watts for the given values, higher if I need more current.

 

[joensd]

If you settle on to the BUF634:
It has got quiescent current of 1.5mA so I´d just use some zeners like millwood suggested.

 

[Adam M.]

That is looking like the way I'm going, buf634 (today, at least...) and I've never used zeners before, sounds like a good way to learn! I'll make sure not to fry myself, I promise.

 

[Adam M.]

I can see 2 different ways to use the zener, in my example I could use a zener with a rating of 23v to bleed off the extra voltage, which is then put in series with the buffer, or I could use a 12v diode in parallel with the buffer, knowing that the voltage drop across the diode will be 12v, which I could tap from before and after the zener. Which is the preferred method?
Thanks,
Adam

 

[joensd]

Hi Adam,
if you don´t have a book about electronics have a look here how it´s done with zeners.
Basically you need a resistor that feeds and at the same time protects the zener diode with appropriate current.
And yes you parallel your load(buffer) to the zener then.

Take your time to read and understand cause you might find in the end it´s easier(cheaper) to just get a 7815 and a 7915 to apply juice to your buffer.

 

[Adam M.]

I still have a little while before I start building, so I have some time to look over the spec sheets still. (I have a DIY sub and Rear speakers to finish before I start the amp). I may try both solutions, the 7815/7915 pair isn't expensive, neither are diodes and resistors, and it'd be a good learning experience. Thanks for all of the help and patience.

Do you have any suggestions on good books? I have books on digital circuits and AC circuits by JR Codgell from when I took electronics classes, but they were a little heavy on theory and a little weak on practical application.

Thanks,
Adam

 

[joensd]

I´m sorry I just have a good german book at hand so I´d suggest you do a search here in the forums for english-written books.
A lot of what you might need to know about for example zener diodes, discrete buffers, opamp circuits can be found on the net.
If you plan on using a specific part always download the datasheet cause there are extensive example circuits in and usually it´s well described how it works. (for example 7815/7915/BUF634)
Just google for "BUF634 datasheet"...

I attached an example of a way to use a source follower as buffer.
Basically it´s just Q4 and R43 doing the job.
Experimenting with those discrete buffers always reassure you don´t have a substantial DC-offset at the output before you connect your gainclone AND speakers. (or use a 10uF output cap after the buffer as alternative)