Posted 20th March 2013 at 03:18 AM byabraxalito Updated 20th March 2013 at 03:30 AM byabraxalito(Added FR)
Passive filters rock for SQ, no doubt about it but I'm still curious how good sounding an active DAC I/V post filter might be. So I've figured out an almost equivalent FR active version of my 7th order LC elliptic filter. This active elliptic has been designed using LTSpice's FilterCad program giving the pole/zero positions, then the Williams handbook of filter design helped me translate those numbers into a working circuit. Its using what Williams calls the VCVS 2nd order section based on a twin-T network to realize the zeroes.
My first attempt at an active elliptic filter was using gyrators but that proved very hard (practically impossible) to get stable with CFB opamps due to their HF gain peaking. VFB opamps I ruled out at the start for inadequate SQ - its not hard to make gyrators stable with them. Hence this approach which promises to work with CFBs though I'd guess I'll probably need to add series Rs between the stages in practice. Nothing built yet but thought I'd...
Posted 19th March 2013 at 06:02 AM byrjm Updated 20th March 2013 at 01:54 AM byrjm
Mid-range 1970's stereo receiver.
I was curious to find out a) what the phono circuit was, and b) how tight the RIAA response might have been.
The answer is "four transistors" and "pretty damn good", respectively.
We are impressed. These Japanese engineers knew a thing or two. I would like to see some of these old circuits resurrected as discrete phono stages with modern components to see just what they are capable of.
Results shown below, together with the LTSpice .asc file you can use to play around with this yourself.
First attached image shows FFT for the rectified DC (green), reference voltage (red) and X-reg output (blue) for the designed-for 10 mA output (top) and a more punishing 100 mA (bottom).
Second image shows an LTSpice screengrab for the LT1086 with bypassed adj pin under comparable loading. Input voltage in blue, output in green. This is a reasonable approximation of a "good" IC regulator.
Last image shows a plot of the exported LTSpice FFT data for the X-reg and the LT1086-12V (Cin 1000uF, Cout 100uF) both at nominal currents of 10 mA. The LT1086-12V is a reasonable substitute for a generic LM7812, i.e. a "bad" IC regulator.
A typical op amp will have sufficient PSRR to mop of the residual noise from the bypassed LT1086. The fixed LT1086-12V, on the...
There is a consistency across several decades and cultures that high efficiency speakers sound more dynamic than low efficiency. That's a common subjective assessment, so we might ask "why?"
I'm not going to chip in on that thread because I have bigger fish to fry, but ISTM they're all barking up the wrong tree. That's because the question itself isn't quite posed correctly even though its a great start. Nobody listens to speakers alone, they're always powered by amplifiers. The answer to dynamics lies in amplifiers, not speakers. Put another way 'Its the electronics, stupid!'.
Higher efficiency speakers tax amplifiers (here meaning their power supplies in the main) much less - so there's no way to do an apples-apples comparison...
Posted 3rd March 2013 at 02:58 PM bywlowes Updated 22nd September 2014 at 01:05 AM bywlowes(Update - Bulk foil resistors)
The amps continued to run in and proved to be keepers. Well worth doing some final spit and polish. There was a little hum on one amp and some noise / hash on the tweeters of both channels. Nothing major, but certainly audible at 1 foot.
Using AC filament supply for all but the input tube, I figured I should re read the best practices on wire dressing, hum pots, virtual center tap etc.
Looking back there were some spots to improve.
1. tight wire twist on path from bridge rectifier to first caps. Done, no real change.
2. tight twist on b+ supply to input tube plus added BG 3uF 250v cap right at the tube. No change to noise but love the improvement in sound
3. tight wire twist from psu to b+ on all power tubes plus BG 10uF 250v on each power tube. Like the input tube the BG brought an immediate lowering of noise floor that really brought a finishing to the SQ. Quiet background, micro details and just a smooth rich sound. Not colored, just the way it should...
As many of you know, i am using the Texas Components TX2575 Bulk Z-Foil resistor for a long time
in important circuit positions, especially as I/V resistor.
Recently i stumbled over a short recommendation from Thorsten Loesch.
Originally Posted by ThorstenL
I would second Rohpoint, Neohm less. Rohpoint are truely excellent. For my first TDA1541 Non-OS DAC (in 98) I handmade bifilar wound Non-inductive 25 Ohm resistors.
In susequent builds I compared to Rohpoint (which I have used extensively and found no reason to handwind resistors.
In my commercial designs we use a specfic type of SMD resistors after auditioning tons.
Some SMD resistors are really bad, some are so-so and some are most excellent, though no patch for a Rohpoint Squaristor GR102.
Because i know Thorsten as a man who had the perfect sound in mind,
i ordered one pair 30R to try them in my Tube-I-zator.
Posted 19th February 2013 at 07:24 AM byabraxalito Updated 26th February 2013 at 03:09 AM byabraxalito(Added zoomed passband plot)
I've only simmed this so far, no listening tests. The aim here is to have a passive filter which works into a line level impedance (i.e. 10k or greater) and which gives a decent amount of RF rejection without compromising SQ. I plan to incorporate it behind the RCA input jacks of my chipamp.
Design-wise, it started life as a simple 2nd order filter with single series L and shunt C. But when it came to wind the inductor I didn't have a wire diameter small enough to make the required value (44mH). So I split up the inductor into 2 * 22mH and then couldn't resist hanging a cap off the centre tap. The result has turned more into an over-damped elliptic with a higher corner frequency than the original, but with an impeccable phase response to 20kHz. The elliptic-type plot comes about because of the SRF of the two inductors (around 350kHz).
Since fas42 remarked on the phase performance, I've added the group delay plot for the passband. Almost pure time delay of...
The subject of this box has come up before, after a 3rd reply to queries, this blog post,
One of 2 Fostex factory recommended cabinets. 10.5 litre tuned to 61 Hz. The other is not very good either.
From the sims you can see that with this box any bass response you have will be way down which will exaggerate the FE126's forward midrange and the 7k peak.
The top curve is half-space (ie what you would get if you mounted them flush in a wall or approximately what you would get with BSC added). The lower curve is with no filters and a simulated 6 dB of baffle loss. You would need a 2-stage filter to give any semblance of flat response. You would lose some 12 dB of sensitivity and then you run into the very low xMax which means that for...