Bonsai,
Curious why parallel and not series?
With the newer metal films maybe the whole Vc issue is gone, but in the old carbon days I thought the higher the resistance the higher the voltage co-efficient, similarly it would appear to be moving in the opposite direction of equal value resistors to form the divider (not that I think it much matters with todays resistors).
Thanks
-Antonio
I just used the low-Z balanced output of the AP for every test.
I could have done a much more tedious job of matching the input devices -- if you have them it's worth the effort as you can get below -100dB without a lot of sweat. I just wanted to demonstrate that the results are very good with a modest effort of matching.
The unit i am planning to put in the Hammond extruded case has both the bipolar output devices matched for hfe as well as the input transistors.
I am using Bergquist thermal "sheet" to mount the output devices to a piece of aluminum sheet. Seems more than adequate to dissipate the energy.
I could have done a much more tedious job of matching the input devices -- if you have them it's worth the effort as you can get below -100dB without a lot of sweat. I just wanted to demonstrate that the results are very good with a modest effort of matching.
The unit i am planning to put in the Hammond extruded case has both the bipolar output devices matched for hfe as well as the input transistors.
I am using Bergquist thermal "sheet" to mount the output devices to a piece of aluminum sheet. Seems more than adequate to dissipate the energy.
I just did a quick check on the non-eBAY or wholesale places.... Mouser, Newark have them in stock at retail prices of about a dollar each. But if you want to get close matching and even lower thd, go direct to the Asian sources - often on eBay - to get super low prices and buy a bunch to find tightly matched pairs of each and every transistor compliment.
PS - I usually dont like devices mounted which create high capacitance to a noisy ground (chassis). You can get oscillation, reduced crosstalk, higher thd, higher noise, reduced bandwidth, rfi and who knows what else. So, float your heatsink from the chassis as well as a precaution and for consistant max performance results. I used individual heatsinks on each transistor. Worked fine and was easy to assemble without thermal isolation pads needed.
PS - I usually dont like devices mounted which create high capacitance to a noisy ground (chassis). You can get oscillation, reduced crosstalk, higher thd, higher noise, reduced bandwidth, rfi and who knows what else. So, float your heatsink from the chassis as well as a precaution and for consistant max performance results. I used individual heatsinks on each transistor. Worked fine and was easy to assemble without thermal isolation pads needed.
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Parallel-series, is what one finds in some high end amps of recent issue, addresses both flaws.
I should hope so as at 1v output the THD is rather high.
Care to show results at say 8v 20khz output levels ?? Yeah yeah I know youll be deaf by then but some of us do indulge at those high volumes.
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You may want to re-read the scale. Harmonics are orders of magnitude below audible.
Homemodder,
Interesting but what flaws are better handled by parallel resistors?
Thanks
-Antonio
But what would be your preferred output transistors in this circuit? (low Z HP around 40 Ohms)
Power handling is the same whether you have two parallel or two series: each takes half of total power (assuming equal resistors).
Accuracy also the same.
Induction is double in series and half parallel, but capacitance is double in parallel and half in series.
Voltage coefficient effect is half in parallel.
So it all depends. If you are concerned about voltage effects go for series - each only sees half the voltage.
jan
Jan,
The above conflicts I assume a typo and you mean Voltage coefficient is half for the series case.
Thanks
-Antonio
When you place two resistors in series the inductance increases if they are inline. If you double them back to form a "U" then the inductance decreases! So using two in series can result in lower capacitance and inductance!
However an important issue is thermal matching. Due to tempco there is some third order distortion present. If the feedback voltage dividers are run at vastly different power levels then the thermal lag between the distortion can result in a small level of 9th harmonic distortion. So using a 4R/R divider offers a bit of an improvement.
However an important issue is thermal matching. Due to tempco there is some third order distortion present. If the feedback voltage dividers are run at vastly different power levels then the thermal lag between the distortion can result in a small level of 9th harmonic distortion. So using a 4R/R divider offers a bit of an improvement.
Yes a very awkward statement of me, sorry. What I meant to say is that voltage coefficients effects are half because the voltage is half; the voltage coefficient itself, - the change in resistance per volt across the resistor - is a property of the resistor and doesn't change with the circuit.
jan
Speaking of output transistors -- the MJE200/210 hfe will vary from ~150 to 210. I found that the 2sb649ac/2sd669ac varied somewhat less. These have a higher ft than the On-Semi devices.
I think that the -3dB bandwidth which Richard stated in the article (25kHz) is extremely conservative as mine goes out quite a way...
I think that the -3dB bandwidth which Richard stated in the article (25kHz) is extremely conservative as mine goes out quite a way...
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I find the largest limitation is not amps and preamps but the source material. The HD downloads and a great DAC give the best sound I've ever heard to date. Really lets you know just how good your amps and preamps are capable of sounding. With this headphone amp, you will hear the closest sound to a master possible.
[ at the present time, i use Senn HD800 and HiFiMan HD500 mostly... a few others are here but not used as often]
The circuit values are not optimized for high voltage output... just for headphone levels. And for low Z (at least down to 30 Ohms) which few if any op amps can do by themselves. Today we dont need much more than a couple volts rms to drive anything to full rated output. Using similar topology and only 20db global negative feedback, levels of under .001% at 22 v p-p output was obtained. Note: this was in my TAA article back around 1980! But that was into a high Z. Levels of this amp are in the -110-120 dB range into 30 Ohms and dont forget -- DC coupled without servo and no drift.
In case anyone is wondering, I have used many sim programs and have MicroCap as my favorite. However, this circuit was not developed using any sim programs. Sim programs are great learning tools and useful for some things, but after awhile you just dont need it anymore if you've done enough circuits. This one just isnt that complex.... very refined, yes.
Other than that --
Thx,
Richard
Jan,
Yes I know what you meant, thanks.
But if you consider the 2 series resistors as one equivalent R, then the voltage coefficient for this R is effectively half of each of the individual's.
Simon,
How does doubling back decrease the capacitance?
It would seem to me that doubling back would add a small lead capacitance as the potential extremes are closer to each other.
Thanks
-Antonio