Balanced input buffer

[TheShaman]

Hey all,

I'm looking for recommendations on balanced input buffers. I should note I'm open to both discrete and opamp based designs.
Excellent bench performance, low cost & part count would be some desirable features.

The buffers are going to be installed on a power amp with low input impedance.

Thanks in advance for your input (buffers)!

[TheShaman]

Some candidates I could double-up for balanced operation:

Opamp based:

• SSB01 Sjöström Super Buffer (I like the fact that it uses SMD - high precision, easier to solder, compact)
• Bonsai's (Andrew C. Russell's) Universal Class A Buffer (probably the 4562 version)

Discrete:

• Pass B1
• AMB's A20 (JISBOS-based, already have PCBs and parts for 2 pairs of these so it's sort of the easy way out)
• Lineup & Fotios Discrete opamp (BISBOS)

[homemodder]

Since you have the Jfets for the A20 I would also suggest you look at JLH discrete buffer, just 2 jfets and 2 bjts, very simple but excellent performing. Its close in design to A20 but simpler but no voltage gain possible. Used in some Krell models.

[peranders]

Originally Posted by TheShaman

Some candidates I could double-up for balanced operation:

Opamp based:

• SSB01 Sjöström Super Buffer (I like the fact that it uses SMD - high precision, easier to solder, compact)
• Bonsai's (Andrew C. Russell's) Universal Class A Buffer (probably the 4562 version)

My SSB01 is very discrete... as you can see and you can also get those for free. See my blog.

[twest820]

Fully differential or single ended? If the latter I've used the LME49710, LME49713, and LME49990 in standard differential op amp configuration. Can't hear or measure any difference in the amp's output between them---if you want low cost the LME49720 and LME49740 are logical optimizations. If fully differential I'd default to the LME49724.

Susumu has a nice line of precision resistors in 0402 up to 1206. Here in the US their 0.1% are USD 0.40 to 0.62 in DIY quantities, depending on package size.

[TheShaman]

Thanks for the input guys.

homemodder, I knew there'd be a JLH design that's missing from my shortlist. Yes I already have JFETs, matched and everything. Thanks for the heads-up.

peranders, I might take you up on that offer. I was thinking of donating to Diyaudio anyway.

twest820, yes, I'd rather go fully differential if I could. It's just that those are harder to come by than single ended ones. Of course I might end up implementing a Datasheet circuit..
As for resistors, I was thinking MELF but thanks for reminding me of Susumu!

[Bonsai]

Theshaman,

if you are simply looking to buffer a balanced audio signal, you can probably also just use a 4562 and run both halves unity gain. What is your load?

My buffer design you refer to runs in class A, but if your load is not too heavy, you can also do this with just the op-amps as well.

Anyway - good luck with your project!

[TheShaman]

Load is not that extreme.. The power amps I'm talking about have a "stock" input impedance of around 8k.
Most preamps or even an adequately robust DAC output stage should have no trouble driving the power amp directly. I'm only looking to buffer it to make the input sort of more... "universal" (I assume some folks will try to drive it directly from their source or use some silly high output impedance preamp etc).

[Bonsai]

Then a good approach here will be to use a 4562 set for unity gain. This opamp will easily drive 600 ohms at <10ppm distortion out well beyond 20KHz at 2VRMS output. Remember to put a 47 Ohm resistor in series with the output(as close to the opamp as possible) and to decouple the supply rails.

You wont need the full class A buffer for this, but if you want to make sure you are in class A with the 8k input impeadance of the power amp, I'd tie a 2.2k resistor from the output of the opamps to the -ve rail.

[TheShaman]

Thanks mate! I already have National stuff on the way and some 4562 should be in the package.
I was....ehm... inspired by the input stage Hypex use on their UcD amps.
I'll also try the class A trick you suggest.