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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 8th September 2016, 05:24 AM   #41
pidesd is offline pidesd  Canada
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Quote:
Originally Posted by pidesd View Post
it's a cascode device, so that M1 sees a near constant drain-source voltage, to reduce distortion.

as to how it achieves that exactly is a bit beyond me though....
not that i doubt it has a positive effect...i'm just wondering about the ''mechanic'' of it
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Old 8th September 2016, 05:39 AM   #42
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
See how cascode works in this video https://www.youtube.com/watch?v=Op_I3Ke7px0
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Old 8th September 2016, 06:13 AM   #43
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
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Originally Posted by Tham View Post
Hi All, I am really grateful to Salas for including me as a beta tester for DCG3 project.

I started sometime early June when I received the schematic. With a lot of help (and patience) from Salas, I have managed to put together my working DCG3 prototype and has been tweaking/listening to it the last month.

Let me state upfront that this is my most eagerly anticipated DIY project. I set out to build it double mono, power it with V1.2 main rail and with LDR volumn control, the result is a great sounding and most satisfying preamp in my system.

Let me attempt to describe how it sounds (subjectively in the context of my all DIY system); compared to my last preamp which is based on a famous buffer stage/LDR attenuation/V1.2 psu, it sounded faster, equal to if not more transparent, has better resolution, more detailed, better tone and image density, and exhibit tight control when the music gets complicated.

I strongly recommend everyone here to try this out in your system.
Tham hi, you are welcome. Beta testing feedback and implementation in self styled construction is always the proof of the pudding.
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Old 8th September 2016, 06:48 AM   #44
Jay is offline Jay  Indonesia
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Originally Posted by Salas View Post
See how cascode works in this video https://www.youtube.com/watch?v=Op_I3Ke7px0
Thanks Salas. So the main objective is to increase the bandwidth???

We tried to reduce Vgd such that the effect of Cgd in limiting the bandwidth is also minimum? Or, you aim for some kind of a "sweetspot" Vgd or Vds for that particular hexfet?
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Old 8th September 2016, 07:38 AM   #45
analog_sa is offline analog_sa  Europe
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Salas DCG3 preamp (line & headphone)
Very nice indeed. Would it be possible to use a shunting, rather than series muting circuit?
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Old 8th September 2016, 07:51 AM   #46
Salas is online now Salas  Greece
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Very nice indeed. Would it be possible to use a shunting, rather than series muting circuit?
I avoided that because in case of strong transient the robust output stage is capable of passing strong current to the ground through the relay contacts. But it can be done shunting also.
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Old 8th September 2016, 07:57 AM   #47
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
Quote:
Originally Posted by Jay View Post
Thanks Salas. So the main objective is to increase the bandwidth???

We tried to reduce Vgd such that the effect of Cgd in limiting the bandwidth is also minimum? Or, you aim for some kind of a "sweetspot" Vgd or Vds for that particular hexfet?
M1 Vgd does not move at the real voltage gain rate of the duo so its Cgd does not appear multiplied. M2 shields M1. The weak JFET needs to drive the big MOSFET's capacitance. Without help of another stage in this case so to keep things as straightforward as possible. So we help reduce that Miller and driver induced THD along that. Bandwidth is good and we even spared some in the end but mainly we wanted to decrease THD.

Time for a circuit description.

R1 C1 are forming an input filter. Volume pot is connected at the input adding to R1 but when at max (even with no source connected and zero resistance added by the pot) the circuit must still remain stable. So R1 is required to form RC. At max with 50R source impedance the slew rate of this circuit can be as high as 25V/uSec even after input filtered and compensated (C2).

J1 & J2 form a long tail input pair actively loaded by Q1 & Q2 mirror. VR1 is used to null the DC offset before U1 DC servo is put in place. So to work very little, against thermal drifts only, not against a residual mismatch. J3 is a low capacitance RF type JFET constant current source biasing the LTP.

The signal path directly continues from J1's drain to M1's gate. M2 cascodes M1. R9 R8 bias M2 resulting in its using half the positive rail. C3 forms RC filter with R9 to keep the gate voltage bias clean. Low capacitance is key so to can directly drive the output stage from just a JFET with little distortion as already mentioned. Its a delicate balance situation because there is no intermediate stage. The semiconductors hands on choices matter. With Fairchild fastest TO-220 MOSFETS wasn't as good for instance. Various JFET change the THD trends also. Higher rail voltage keeps making the MOSFETS less capacitive up to 25V or so. Due to dissipation reasons for all semis but especially the high bias output ones, and not to add special regulators for U1 and the relay, a sweet spot was chosen.

M3 is a constant current source for the output stage. Having CCS on both input and output stages offers PSRR also. Its a fast depletion mode (self biasing) MOSFET. R10 sets its current. Bias >100mA can be chosen with M3's IDSS being the limit (150mA minimum). This mA region is also the M1 M2 zero tempco region. Bias stability tested solid indeed with moderate sinking. R6 R7 are low impedance feedback network to the other LTP input. Low impedance is necessary not only for low noise but to also keep this particular circuit stable. In most cases the burden of driving that network is heavier than what the preamp will be called for to drive on its line output. By the way with the more capacitance of output interconnects this preamp sees the more its phase margin. I.e. long cables no worries. Tested with 10nF capacitor even. The interface bandwidth is seriously curtailed in such an extreme though.

U1 picks the output through R12 C4 low pass filter. Integrates via C5 and drives counter-DC corrections through R14. Working at about 2.3HZ proved good. Many U1 types were tested.

The output from M2's drain is interfaced with a lab standard 50R to line output. Another route goes to headphone output. There 0R with a jumper is the technically neutral thing to be used or a judicious additional Rz value that under-damps some headphones which their users may describe as "dry". Caution though, anything more than 10% of the nominal headphone impedance for Rz will interfere with its overall response more than +/- 1dB. Unless someone opts to intentionally color it more.

Because no capacitors are in the signal path a relay system timed to 5 sec delay by Ra Ca lets no DC settling bump during power on to get out. It also disengages quickly during power off. Dc Db Da LEDS drop the rail voltage to 12V relay safe working range territory. And you know its on. One of them can be actually brought to the front panel as "ready" sign because they light up only when the relay clicks.

Adding the schematic again for ease of reference.
Attached Images
File Type: png DCG-3_Sch.png (26.7 KB, 4098 views)
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Old 8th September 2016, 09:33 AM   #48
zorch is offline zorch  United States
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Default Count me in for the DCG3! I have your Simple Folded Phonopreamp and love it!

Quote:
Originally Posted by Salas View Post
Hi all, this thread is about a JFET & MOSFET preamp I am cooking for sometime now. Its two stage single ended Class A circuit.
It was developed on the bench involving much experimentation with topology and a range of active parts. Simulations came second.

Some speakers are insensitive, some amps also, or a source is relatively low output like a phono stage. So a line preamp with gain is good to have.

Its named DCG3 because it has DC servo and three MOSFETS. Its standard gain setting is also times three.

The main schematic is attached

Some THD plots when driving 1.5kOhm line input and various headphones are also attached.
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Old 8th September 2016, 11:10 AM   #49
Tea-Bag is offline Tea-Bag  United States
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Salas DCG3 preamp (line & headphone)
Default First round eval.

Quote:
Originally Posted by Tham View Post
Let me attempt to describe how it sounds (subjectively in the context of my all DIY system); compared to my last preamp which is based on a famous buffer stage/LDR attenuation/V1.2 psu, it sounded faster, equal to if not more transparent, has better resolution, more detailed, better tone and image density, and exhibit tight control when the music gets complicated.

I am using the DCSTB power supply shown above, and a ALPS pot on a In-Select board. Although I want more time for evaluation in my best setup (I listened for about 3 hours this weekend). I can concur with the the 'apparent speed' aspect of it. I also noted that the precision of the image in the sound-field, at least from a left to right perspective is very tight. To get that tightness on a DCB1 for instance, in my system, it has to high biased on the hot-rod side. I am still studying image depth, and tonal qualities.
What I can say is that well done recordings, such as those from Mobile Fidelity Sound Labs, sound extremely transparent and detailed. It does not sound fatiguing, but it does sound intense. You then compare to say default web-browser HIFI Tidal running through USB, and it's not the same. Same with poor vinyl, it just shows the weaknesses or strength of the source recording. I have not done a lot of listening via the headphones at this point, as it just not my preferred listening style. But I do have another headamp to try that I hope to compare to later.
I try not to make attempt to oversell any boards that Salas and I may produce. But I will give my honest evaluation of it. And I need more time with it, more recordings. And more spoken voice, either through running movies through it, or more slowly sung material. To me this is the better test . A voice that you know well. A lot of gear will add a 'signature' sound to this. Really good gear should not add too much.
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Old 8th September 2016, 12:20 PM   #50
Tham is offline Tham  Malaysia
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Just a bit of my listening experience with regards to imaging capability of DCG3...

I put on Amused To Death CD yesterday and the effect is akin to listening through a huge pair of headphone. A totally enveloping sound field; very good sense of depth, well separated images front and back and even above and behind the listening position, and all those (surround) sound effects contained in the recording are well spread and mostly discernible.

It is one of the most difficult test I have put the DCG3 through so far and I must say that this CD has never sounded better before.
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