Balanced IC PreAmp

[Samuel Groner]

Hi

I've been thinking about a balanced IC Mic Pre and that's what I got: [removed]

Some comments:
* a final design would need some more features like phantom powering, pad, gain switching/setting and perhaps an output buffer pair.
* the first gain stage will be very happy with gains between 6 dB and 40 dB; other gains could be realized as well with suitable changes.
* the second stage will best be used between 0 dB and 20 dB - this gives 20+40+20-6=74 dB max. gain, sufficient for most situations! The inverting configuration was choosen because of higher slew rate and stability.
* gain setting of the first stage is done by a log pot or a rotary switch. The second stage is probably best switched by relais. A ganged control of the two stages could be realized with a multy deck rotary switch.
* I would probably use NE5534A; there are other appropriate ICs.
* depending on the required max. output level the output XFMR could be anything between 1:1 and 2:1; the 2:1 as shown reduces the load of the ICs which is probably a good thing.
* three of the four 1000 uF caps are a bit oversized (the shunt cap in the first gain stage should be as big as shown); I opted for these values for simplicity.

Now my questions:
* what DC offset do I have to expect for the output transformer primary? I expect most offset to be common mode due to some matching of the ICs; is this a good assumption?
* is the 1000 uF cap that shorts the two 6k8 bias compensation resistors for lower noise well applied or should I use two caps (each going to ground) instead?
* I've seen some 5534-designs that put a 1k in front of the noninverting input (for stability and protection, I think). Would this be a good addition for this design?

Any comments appreciated!

Samuel

 

[bcarso]

Hi Samuel,

Philips (http://www.akustische-kunst.org/audio/datasheets/NE5534.pdf) says 500uV typical for the input offset voltage, although you can trim this to zero at a given temp. The worst-case offset current supposing that each second-stage amp is opposite in polarity from the other would add as much as 5.4mV differentially for the NE part over temperature. Your actual results are likely to be better of course.

The 1000uF cap as it is now effectively ties the two inputs and their resistors together but does nothing to bypass noise. Use two separate ones (and 1000uF is awfully high it would seem).

Philips says limit the input currents to 10mA---there's a pair of diodes there to prevent reverse-biased breakdown, with its attendant noise degradation, of the E-B junctions of the input transistors. I doubt that you will get that 10mA from your source unless someone plugs the preamp input into some amp output. You could put 1k there at the n.i. input pins but you will spoil your noise performance a bit. I would just be careful. But the second stages at 20dB gain (each?) will have 680 ohms as the input R, which means a bit over the recommended max current when driven hard enough by the previous stages. You would be protected by the bias current balancing R's except for your big bypass C's, which will pass lots of current at audio frequencies. So, since you are out of noise problems usually by the second stage, you could have a portion of the bias I balancing R's unbypassed--say 1.2k of your 6.8k total, or 1.2k + bypassed 5.6k, a combination that is conservative and uses standard values. Now the protection diodes in the 5534 will see at least 680 + 1.2k, so even for a 16V output swing from the previous stage you will limit I to <7.8mA. You could also limit I from the previous stages by putting some R between the first stages' output pin and the existing circuit as drawn, thus enclosing the I limit R's within the feedback loop---this would avoid any noise disadvantage of the other approach, although it would very slightly limit overall output swing when the second stage is run at 0dB. I am not sure what the effects on transient and frequency response would be however.

Or, again, you could just be careful not to overload!

Brad

 

[PRR]

> The inverting configuration was choosen because of higher slew rate and stability. * I would probably use NE5534A; there are other appropriate ICs.

If you use the internally-compensated 5532, I don't see where you get better performance inverting.

> * depending on the required max. output level the output XFMR could be anything between 1:1 and 2:1; the 2:1 as shown reduces the load of the ICs which is probably a good thing.

With 1:1 and a true 600Ω load, each chip sees 300Ω which is a pretty stiff load for 5534/5532. With 2:1 you will get ample but not stunningly high output. We saw a German mix-stage here recently that used four 553x amps in parallel-push/pull.

I would omit the resistors on the +In of the second stage. If you use 5532 the bias errors in the resistors on the -In will mostly cancel, and it is unity-gain for DC.

> what DC offset do I have to expect for the output transformer primary?

I'd guess you will see a few mV. The DC current in the transformer, assuming an ungapped E-I core, should maybe be <1% of the peak rated AC current at the lowest frequency. If the core is rated 100mW (+20dBm) across 600Ω at 20Hz, that's over 10V peak at 600Ω or 18mA; take max DC current as 0.2mA. If you have 2mV of offset and need <0.2mA current you need >10Ω DC resistance. You will probably have more than that in a 600Ω winding, but you do need to check real-world offset voltages and DCRs to be sure all the estimates and approximations are not stacked against you.

 

[henk]

Hey PRR , it would be nice to have a design that has four parallel 553x , can you give more information about the design , could you make a design (paper+pensil) and put it in this forum?

I would like to see something like that , because i have never seen something like that and would like to see it.

So i do learn something , so i stay young?

 

[bcarso]

Samuel, another thought regarding the input current limiting: you could just put 1N4148's across each pair of inverting/noninverting inputs throughout. Their C's are low enough that at these impedances they won't affect stability, and they will handle 100mA. Then there is no voltage swing or noise penalty from adding resistors.

Brad

 

[Samuel Groner]

Thanks for all replies!

If you use the internally-compensated 5532, I don't see where you get better performance inverting.

You're right, slew rate is identical. But stability would be better, no? At least gain is way down above a few hundert kHz whereas it drops to unity for a noninvertig configuration up to a few Mhz, which seams not helpful for best stability.

I would omit the resistors on the +In of the second stage. If you use 5532 the bias errors in the resistors on the -In will mostly cancel, and it is unity-gain for DC.

The 1000uF cap as it is now effectively ties the two inputs and their resistors together but does nothing to bypass noise. Use two separate ones (and 1000uF is awfully high it would seem).

Both solutions convince me. Don't know which one I will choose.

Samuel, another thought regarding the input current limiting: you could just put 1N4148's across each pair of inverting/noninverting inputs throughout.

Thanks for the suggestion. In fact I was more initerested in the stability issue; if the secondary winding of the input XFMR has 1k DC resistance, does this ensure the same phase margin as with a 1k resistor added?

Hey PRR , it would be nice to have a design that has four parallel 553x , can you give more information about the design , could you make a design (paper+pensil) and put it in this forum?

Check this schemo:
475-2B_Schematic.pdf
It can easely be expanded to four 5532.

Samuel

 

[bcarso]

Samuel, I think PRR is alluding to the tendency to inherent matching of the two sections of a dual op amp (viz. 5532). Since they do tend to be pretty well matched, omitting the resistors in the n.i. connections of the second stage probably will work fine. I would still use the 4148's if you want bulletproof performance.

As far as the stability margin issue goes, I don't believe this family of op amps has a need for seeing significant resistive Z on the n.i. inputs in your configuration. There are some amps that do when used at very low closed-loop gains, case in point the AD797 (as I found out to my chagrin when I was trying to use them in a "blameless" buffer application and attempting to get true 24-bit S/N performance). In the 797's case it is more of a parasitic oscillation arising from the very high gain bandwidth and low rbb' of the input devices, interacting with inevitable lead inductances I believe. AD does warn you about this in the fine print btw.

Having said that, the winding R should behave much like a lumped external R for most purposes.

Some amps will even work better (i.e. be more stable) the lower the Z at the n.i. inputs. Of course this is very much in contrast to the instabilities resulting from driving capacitive loads at the outputs, or having too-high feedback impedances interacting with input capacitances.

Brad

 

[Samuel Groner]

Brad, thanks for the clarification!

Samuel