Preamp design question -- where to put buffers?

Hi!
I started the design of my preamplifier. I decided to go with ESP projects since they seem to be of good quality and easy to build.
I made a basic block diagram of the preamp and I have a couple of interrogations before I begin the pcb layouts.

Everything will be controllable with a remote control so the line in selector is made with Analog Devices SSM2404 switches. The Phono input goes through a Phono preamp before the switch selector input. Then, in order, the signal would go through a Tone control, DOZ preamp or Headphone amp, and the Volume control. Everything is 'bypassable', so I will put SSM2404's to allow the signal to
There is also a Tape-out output.

-I'm wondering where I should put buffers? Should I put one
before each 'block' in the diagram? Does anyone know a good buffer circuit?

Thanks for any input
David



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you want buffers?

I'm not sure what your needs dictate, but here are a couple of VERY simple ones that have worked well for me over the years. Tweak to your heart's content!

Good luck,
Michael

PS: Since you'll have to use +/-12V rails analog rails because of the SSM2404, you may want to look at the AD8610 op amp as a buffer since its max rails are +/-13V.
 

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paulb

Member
2001-06-01 4:53 pm
Calgary
First off, I'd suggest a buffer on your tape output. You really should isolate any loading effects from your tape deck's input.
You mentioned everything is remote control - what are you using for a volume control? You may need to buffer that. Or how about moving the volume control in front of the preamp section?
Some of the SSM24xx switches suggest you should drive into a virtual ground, meaning you'll need an opamp there. Check the app notes on the 2404 carefully.
 
You are right for the tape out, I'll buffer it.

You mentioned everything is remote control - what are you using for a volume control? You may need to buffer that. Or how about moving the volume control in front of the preamp section?

This is another issue I wanted to address. I have two choices

1) Use a digital potentiometer. The one made by Crystal seem to perform an analog to digital conversion which I'd like to avoid. Maxim-ic (Dallas semi.) has a lot of digital potentiometer. I thought the DS1807 was interesting. I'd like to hear comments about those potentiometer...Maxim states they are VERY good.

2) Build a logarithmic pot with ssm2404's and resistors. Elektor electronics published a circuit a while ago that was described as : Very good quality potentiometer.

As for the position of the volume control, I chose 'after' the preamp because ESP did it in his preamp. Do you have any recommendations about this?


Some of the SSM24xx switches suggest you should drive into a virtual ground, meaning you'll need an opamp there. Check the app notes on the 2404 carefully.

I read the ssm2404 datasheet and app notes a couple of time and I can't seem to find any proper way to use the switch. In one of the app note, they build a line in selector. The circuit they propose is complicated and they don't really explain why. They have opamps before and after the switches and they are not only Unity gain buffers. They seem to attenuate the signal before the switches and then reamplifie it. I do not fully understand what is going on there so if anyone knows how to use the switches, please help!!!

Thanks a lot
 

paulb

Member
2001-06-01 4:53 pm
Calgary
gretzteam,
So, where'd your name come from?
Re SSM2404: I used these in my preamp for source selection. My circuit is based on Fig. 23 or 26 in the datasheet: A 5K resistor before, running into a virtual ground opamp. You can extend Fig. 26 to as many inputs as you need (only selecting one at a time, of course). I don't have the input protection diodes but they seem like a really good idea.
Re position of volume control: If it is after the preamp, your output impedance will vary with volume setting. This may not be a big deal if your preamp is close to your power amp and the latter has a high input impedance. Otherwise, I'd buffer after the volume control. If the preamp has a significant amount of gain, putting the volume control afterwards reduces the noise from the preamp as the volume is lowered - also desirable. Your call. This is what makes it fun and why we keep building more and more and more. I like your idea of putting everything in then being able to bypass it.
Re potentiometer: this topic comes up often on this board. I believe there was also some discussion on the Dallas (Maxim) parts at headwize.com. Typing part numbers into the search will usually get lots of hits. Many digital pots can only practically be controlled from a microprocessor, which is probably either not a problem or insurmountable, depending on your electronics background.
Re LM6121 (or LM6221): probably overkill. Really expensive.
 
Thanks a lot Paulb for your help it is GREATLY appreciatted.

SSM2404 : You are right with the figure 26, it is exactly what I need to do. I was looking at the ssm2402 datasheets and the application note is weird...that's why I was confused.
They use a ssm2131 opamp in the datasheet. I can't find any informations on this opamp on the Analog website...it's not even listed in the obsolete part. I don't get it. Do you know a good replacement? LM6121? even if it's expensive...
Also, are you happy with your line in selector? How do you compare it with one that is built with relays?

Potentiometer:
I will control everything with a microcontroller. For me the digital design is the 'easy' part. I just want to make sure I choose the right components...because my analog skills are quite limited. I searched a lot through this forum and I can only find posts of people that would like to try those pots (maxim, crystal, national). Nobody gives clear results of their experience...maybe I should simply try them all...

Buffers:
I found a unity gain circuit in the welbourne labs catalog. I attached the image. Can anyone tell me if it's a good one? Does it worth it to go with this or there are better up to date circuits (this one is from 1990).

Thanks a lot

P.S. My name simply reflects that I'm a disturbed fanatic Wayne Gretzky fan. GO CANADA!!! WE WON GOLD

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