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Old 2nd September 2011, 04:29 AM   #51
EUVL is offline EUVL  Europe
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Incidentally this (what you called Zen Bipolar) is almost identical to the Leach 1999 circuit (2nd in the web page).
Except that he used 2 caps to replace your V2 & V3.

Moving Coil Cartridge Head Amps

Note that he did not ground the common emitter point of the mirror transistors (Q3, Q4).


Patrick

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Last edited by EUVL; 2nd September 2011 at 04:37 AM.
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Old 2nd September 2011, 08:42 AM   #52
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check post #3 about
Simulation of I/U (I/V) converter versions for TI PCM R2R DAC's (e.g. PCM1704) ?"
so as the thread
Best opamp for I/V conversion? (DAC)
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Old 2nd September 2011, 09:17 AM   #53
Calvin is online now Calvin  Germany
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Hi,

had a look at the Leach circuit, which looks certainly similar at first glance.
There are a few points to remark though.
The Leach-pre is intended for usage with very low input currents and bias currents. In that topology the current mirroring Q3,Q4 ´steal´ some of the signal current, which means that not all the signal current swing can be used in the I/V conversion resistor. So either the output voltage is lower or one needs a larger resistor value. My sims showed losses of 10%-20% of outpt voltage.
Decreasing the currents through the collector-resisitors R3,4 is limited by the minimum needed base currents of Q1-Q4. The smaller idle currents through Q3,Q4 -compared to Q1,Q2- demand the use of the common emitter resistor R2 to allow for sufficient current in Q1,Q2. R2 should be two resistors with halfed values and referenced to Gnd (as sketched). Otherwise the input of the circuit will have no DC-reference point, because with a DAC-current output there is no input resistor R1, hence the input may find itself anywhere, but certainly not around 0V. Especially if You use DACs with centre currents like the PCM-series of TI. Without the gnd-reference via R1 Leach´s circuit would cease working as intended.
But there´s a subtle more to the gnd-referencing of the emitters of Q3 and Q4. If not referenced the input impedance -dc as well as as ac- nearly doubles against the ´referenced´ value.
Btw. the two 560k-resistors R1,R2 in EUVLs schematic serve a similar purpose...to reference and centre the circuit to gnd.

Leach´s circuit needs only one floating supply, but since the output impedance is rather high and the bandwidth and output voltage depend on the input impedance of the following device, a buffer stage would almost always be necessary. You can then use the power supplies of the buffer stage to supply the current mirror transistors. Breaking the current path through Q3,Q4 from the floating supply V1 lets all signal current pass through Riv. The output voltage is maximal and calculates to Riv times DAC-signal current.

jauu
Calvin
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Old 2nd September 2011, 09:50 AM   #54
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Quote:
Originally Posted by cviller View Post
Well diy is a hobby... If you are afraid of losing time by not building the best, you'll have to wait forever. There is always something new and exciting.
Yes, but I would have to reach an age of more than a thousand years, to examine all my own and the other circuit ideas.
For this reason, I always need just a pre-selection with help of the experiences from other guys.

Quote:
Originally Posted by Calvin View Post
Hi, had a look at the Leach circuit, which looks certainly similar at first glance.
There are a few points to remark though.
The Leach-pre is intended for usage with very low input currents and bias currents. In that topology the current mirroring Q3,Q4 ´steal´ some of the signal current, which means that not all the signal current swing can be used in the I/V conversion resistor. So either the output voltage is lower or one needs a larger resistor value. My sims showed losses of 10%-20% of outpt voltage.
jauu Calvin
This leads me to the question, which frequency-dependent influence actually occurs through the three capacitances (input-, reverse- and D-S-capacitance) of the IRF MOS-FET from the Pass D1-Topology about
http://www.passdiy.com/pdf/Z-IV.pdf
(second drawing - simplified schematic D1).

Last edited by tiefbassuebertr; 2nd September 2011 at 09:59 AM.
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Old 2nd September 2011, 02:51 PM   #55
EUVL is offline EUVL  Europe
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There are 3 versions of the Leach circuit, Common Base (CB), Current Mirror (CM) and Common Emitter (CE).

Moving Coil Cartridge Head Amps

The one we simulated at the beginning was the CB and not the CM circuit (post #46).

I am the first to admit I know next to nothing about current mirrors and BJTs.
But I could not help wondering :

a) What would be the distortion like if the current mirror has some typical mismatch ?
(e.g. 50mV Vbe, 3% hfe, as in LS312/LS352 or SSM2210/2220)
b) What would be the PSRR of the circuit to noise in V2 & V3 ?

We did some very quick Spice simulations on (a) which revealed as much as 0.02% THD for 5% hfe mismatch !!

With the added components connected to Gnd (V2, V3, Q3, A4), the way the circuit functions has changed fundamentally (Please refer to my article for my interpretation). There is no longer a separate current loop for the DAC i_out through Riv back to Gnd. I am most concerned that the distortion spectrum would become more complex as a result.

I do very much look forward to your experimental results with real devices.


Patrick

PS My archive revealed quite a few BJT IV circuits based on current mirrors, e.g. the famous one from Jocko Homo.

Easy-to-build I/V stage

.

Last edited by EUVL; 2nd September 2011 at 02:59 PM.
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Old 2nd September 2011, 08:50 PM   #56
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Got my issue of linearaudio in the mail today
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Old 3rd September 2011, 02:42 AM   #57
EUVL is offline EUVL  Europe
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Happy reading.


Patrick
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Old 3rd September 2011, 09:32 AM   #58
Calvin is online now Calvin  Germany
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Hi,

while the so called "current mirror common base" stage of M. Leach certainly looks like its function relys on current mirror action, it is not what´s happening here. Usually the input and output of a C-M-stage is taken from the collectors. What we have here in fact is a complementary common base stage with temperature compensation. The difference to the common-base-stage of M.Leachs is in the biasing network. The diode connected Q3 and Q4 are used for temperature compensation and to set the idle currents of Q1 and Q2. This is maybe more obvious in Jocko´s circuit and the ZEN-I/V in #43. When dueal matched pairs are used for Q1,Q3 and Q2,Q4. input offset should not be an issue.
As Q3 and Q4 only serve as biasing devices -e.g dealing with DC working points, no AC- mismatches in Vbe and hfe should only lead to a slight variation of Input offset voltage (Q1,Q2s emitter potential, which ideally is 0V). With the biasing transistors Q3 and Q4 referenced to gnd and due to complementary action the input offset will be small already.
You could add a low-value pot between Q3 and Q4s emitters to null out dc-input-offset. Alternatively a dc-servo could feed the centre tap of two equal low ohmic resistors spanning between Q3 and Q4s emitters.

jauu
Calvin
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Old 4th September 2011, 06:30 AM   #59
EUVL is offline EUVL  Europe
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If they are not current mirrors, perhaps you can explain why changing Q1 / Q4 hfe by 5% increases THD to 0.019%?
(in the CEN Bipolar as you posted in post #43)



There is also a factor of 5~10 more current from i_in not flowing through R-iv when compared to the JFET versions.
Namely it sinks directly to Gnd via the Gnd connection near V2 / V3 / Q3 / Q4.
The comparison was made under the same I/O conditons as yours (2mA input current, R-iv 1.5k).

I am sure you can repeat the results, since we built a Spice model according to your schematics to repeat your results in post #49 first.
When we have some more time, we'll try to find out about PSRR to V2 & V3.


Patrick

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Last edited by EUVL; 4th September 2011 at 06:57 AM.
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Old 4th September 2011, 06:50 AM   #60
EUVL is offline EUVL  Europe
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Attached the Spice model and the "leakage" current of the bipolar circuit.


Patrick

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File Type: png cen-iv-bipolar-current-leakage.png (58.1 KB, 1404 views)

Last edited by EUVL; 4th September 2011 at 06:55 AM.
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