Spice and the art of preamplifier design, Part 2. SPICE. and the art of preamplifier design. Part. 2: New preamplifier designsby. Norman L. Koren. Updated August 2. Vacuum tube audio page .
We described the Dynaco. PAS preamplfier- - its circuit and performance. In this part we present. PAS, designed with the help of SPICE, that solve. The modified PAS preamplifier with purist line stage, (re)built. One of the principal choices in phono preamplifier design is between active.
Dynaco PAS-2/2X Preamplifier. Photo courtesy of James Giske. 3 stereo low level: RIAA phono, NAB tape head. Dyna's first stereo preamp. Find best value and selection for your Dynaco-PAS3-Tube-Preamplifier-Super-PAS-Van-Alstine- search. New listing NEW STUFFED BOARD Dynaco PAS Preamp PC-5A Line Level (PC. DYNACO PAS 3 STEREO TUBE PREAMP W/PHONO - TESTED. DYNACO PAS PREAMP (Models 2, 3, and 3X) PAS 2/3 Owners Manual (full) PAS Repair Guide. Replacing the PAS Volume Control. Removing the PAS Tone Controls. Replacing the PAS Selector Switch.
Dynaco PAS-3 (Vintage) - Preamplifiers -. Stereophile reviewed the AVA-rebuild of the Dynaco PAS preamp back in 1988. Dynakit Parts, dynakit st70, dynaco ST70, dynaco st35, dynaco markii, stereo 35. Complete Restoration of a Dynaco PAS Pre Amp. Labels: Curcio Audio, Dynaco Restoration, Dynaco Tube, Dynaco Tube Preamp, Kiwame resistors, PAS. PAS-Chassis & Parts. UPDATE: Tube Nirvana now offers a full PAS Preamp Kit & SCA-35 Integrated Power Amplifier Kit.
Passive equalization. Too much gain. prior to the equalizer reduces headroom, increasing the danger of overload. The ideal feedforward.
B gain prior. to the equalizer (more for low- output moving coil cartridges) and 2. B. gain following it This isn't really practical with the three 1. AX7. sections available for the phono preamplifier. We could use two sections. B gain, but since no extra. AX7. This could result in an.
Van Alstine Super PAS 3 KIT preamp based on a Dynaco tube preamp. Van Alstine Super PAS 3 preamp KIT Rebuilding a vintage Dynaco preamp Part I This is the Dynaco PAS3 manual. F ellow hobbyists should find it useful to maintain and modify this versatile little preamp. Please use your browsers 'back' button for. Schematic (2 of 2) Page 12 - 'Special' Input and.
A capacitor connected. Miller capacitor after the Miller. Its capacitance must be multiplied by the closed- loop gain of the. RC time constant. This capacitor stabilizes.
RF interference, and continues. RIAA rolloff at very high frequencies. Its value is not critical: 2. F is OK. Before it was added, certain Chinese 1. AX7's (which may.
PAS was switched to. PHONO. Transient response problems in feedback phono preamplifiers. Miller capacitor. C1. M. also gave the PAS a stronger subjective sense of silence in quiet passages. Since the original design has. The redesign started by.
RIA2 from 4. 7. MEG to 2. MEG. Other component values (RIA2, CIA1. CIA2) were determined by trial- and- error. Frequency response (below) is.
Hz to 1. 00k. Hz. All they do is pick up noise.
Place CIA1 on the board at the original. F capacitors. Tone controls complicate. The purist who prefers the simplest. The first two are suitable as PAS modification.
You should consider the second two if you plan. Version 1 used the old tone control potentiometers. Components have been recalculated for improved flatness. Version 2 used widely- available linear. Both versions 1 and. This means that. you may need to replace an internal load resistor when you change power. Version 3 has the huge advantage that it works with any reasonable.
Because versions 1 and 2 are of little practical interest. I've dropped them from the article. I've used it in. my main stereo since 1. It solves the low frequency resonance problem.
PAS in a most elegant way: with dc- coupled negative feedback. Since this affects the bias of TU3, series. R3. C and R3. B are employed so that the bias network consisting. R3. GJ, R3. G, and C3. B, which keeps input grid 3. G at the same dc voltage.
B, results in the desired plate current of 0. A. The overall gain of 9. B) is determined by the feedback voltage. R6. C and R3. C+R3. B. I tried many configurations before settling on this one. Simulations. took about fifteen seconds on my 1. MHz Pentium in 1.
MHz Pentium in 2. Without SPICE I could have never attempted.
If we did nothing to reduce. TU4, the signal at the plate 3. P of input stage TU3 would be. B, which worsen the sonic degradation due to RF interference.
To prevent this problem, we have reduced. TU4 by means of local feedback, which is implemented.
R4. G to junction 4. B between R4. C and R4. B. Local. feedback is extremely stable and delivers all the usual benefits: reduced. It also. improves stability by decreasing the amount of global negative feedback.
In the present modification. CLFB, replacing it with Miller capacitor C3. M connected between. G and plate 3. P. We also add C3. C connected between input. C and ground to shunt RF interference from the output cable.
The validity of this technique was confirmed by. Journal of the Audio Engineering Society, which. Miller. compensation is a superior approach to error- correcting amplifier design. The first gain stage is the appropriate location of this pole because it. This approach. helps stabilize the preamplifier when capacitance is added in parallel. F per foot. This resistor, which. In a non- feedback amplifier, capacitance.
In a feedback amplifier. The addition of this capacitor increases the effective. Frequency. response is nearly unaffected by shunt capacitance up to 1. F. ROUT. effectively isolates the line amplifier from the load. The traces represent the.
CLFB = 1. 0p. F is connected in parallel with R6. C, and C3. M and C3. C are omitted. This RF feedback is worst in trace 1, which. PAS. It is improved by omitting CLFB, and gets.
C3. M and C3. C are employed, as they are in the final design. Ferrite beads. aren't very effective because their impedances are much lower than the. Replacing CLFB with C3. M and C3. C made a definite. My instruments can't measure extreme.
A small 0. 0. 4. Hz peak is observed on first gain stage plate 3. P, but its gain is less. The original PAS has a 0. Hz peak 4. 0 d. B above. Output impedance ZOUT is measured by connecting. VIN to LINE. It is below 2.
Hz. and 2. 0 k. Hz, increasing to 6. Hz and 1. 20k. Hz. Thanks to the stabilizing.
ROUT, it can drive any practical length of cable. Below about. 5. 00 Hz output impedance is dominated by output coupling capacitor COUT. V monster in my mod (Fig. Wima supplier didn't. F rated at 2. 50. V. The sound is superb in every respect.
Between this and The Emperor's New Amplifier. I've become contented- - ruined an audiophile. I just enjoy the music.
It has a. relatively restricted tone control range compared to the original PAS. The 5. 00k dual- ganged potentiometer. Dual- ganged potentiometers are less common than. Mouser. Electronics has one (reasonably priced) in its catalog (stock no.
VW5. 05. in their 2. Since it's not supplied with a knurled. We have used the parametric. SPICE to generate these curves. The tone control range is. B/- 5. 4d. B at 5. Hz and +7. 3d. B/- 5.
B at 1. 0k. Hz. This relatively limited. Output impedance. C3. G is in. the input circuit.
With this circuit you never have to worry about your. If you have already performed. Mouser. Electronics. The recommended power supply schematic is shown near the. Part 1. Note that the three tubes in the phono preamplifier are. V+2. 80). Similarly, the three tubes in. V+3. 20. The power supply electrolytic capacitors.
We have. designed the circuits to minimize this ac current. For example, the ac. TU4 and TU6 in the line amplifier); hence. Connecting. them to separate supply taps would definitely degrade the sound.
Add extra. filter capacitors (1. F or greater; 4. 00. V): one of them above the 1.
X4. socket and two of them close to the rear of the chassis. Mount a. 1. 2. 6 V transformer for powering the filament windings (Radio Shack 2.
The. 1 ohm 2. W (minimum) resistor in series with the secondary reduces output. Install a new 1. 00k dual- ganged audio taper.
Mouser part 3. 13- 2. K. does nicely, but a more accurate precision potentiometer would be desirable.
I used a chassis. The first step was to tear out all components not used. There were several places where three leads had. This would be unacceptable in a. OK in an individual project if done with great. I often use silicone seal to stabilize components, like the giant. F 6. 00. V output coupling capacitors.
A new PC board. design would be very desirable: I would be happy to collaborate with. Keep wires with high- level. Use parallel capacitors to get within 1% of the specified.
RIAA network and 5% or better for the tone controls. Coupling. capacitors are less critical. R1. P through R4. P and follower cathode resistors R3.
C. and R6. C. For example, R6. C dissipates E2/R = 1. Some extra margin reduces the effects of heating. Polypropylene. or polystyrene capacitors should be employed where possible. I recommend. capacitors manufactured by the . The 1. 2AU7's or. SN7's in the output stage draw too much plate current (8 m.
A per channel). for the wimpy PAS power supply. I designed it in late 1. I completed. and shortly before the events that led to my move to Colorado. I still. haven't built it. This contrasts with the dual- ganged. The component tolerances in. Of course I'd. use a much heftier power transformer than the PAS.
Don't expect dramatic bass or. It has. no global negative feedback whatsoever; its circuit is simplicity itself. You need global feedback to get good response out of the 1. AX7. The. 1. 2AU7 was down only 2.
B at 1. 00 k. Hz. The 6. 8 p. F capacitor, C3.
B at 1. 00 k. Hz. The local. feedback on each tube stabilizes the gain. Output impedance is low enough.
F shunt capacitor doesn't cause substantial high frequency. As with my other designs, the net ac current drawn from V+3.
This effectively removes the power supply from the signal path. This is an extreme high end. It can serve as a standard. But the time has come to take a. And whether it sounds as good. Libraries of schematic. SLB and libraries of SPICE.
LIB. You may download the. PSpice examples by shift- clicking here on Tubemods. I'll eventually consolidate it with material in Improved. Part 2.)Since the technique for linking drawings to models is quite complex. Micro. Sim tutorial should be perused. A1 illustrates Schematics circuits that can be used to generate.
Type to Voltage Source, Sweep Type to Linear, Name: to VG1. Start Value to 0, End Value: to - 7. Increment. to - 1. OK. Type to Voltage Source, Sweep Type to. Linear, Name: to V.
V in. Fig. 5 (ultra- linear (UL) mode). A2b, V(%IN+, %IN- )*. V(1. 01)*. 6. is for ultra- linear mode.
Marshall Leach, Jr., . John Bicht interview, . Christopher Paul, . About the author.
Norman Koren, a native of Rochester, NY. BA in physics from Brown University in 1. MA in physics. from Wayne State University in 1. His destiny as a high- tech nomad has.
Boston, Philadelphia, Silicon Valley, San Diego, and most. Colorado, where he worked in research and development of digital. Back to Part 1: Background. Norman Koren Vacuum tube audio page.