Hi All,
I've built a couple of projects from scratch, and for 2010 I'd like to try my hand at a phono preamp. Since it's my first go, I'd appreciate it if I could get some comments, critques, "yeah, but"'s, or what have you. Here's my first shot at drawing something up:
A couple comments on the design. The gain in the first stage is a little low for what you might expect out of a cascode but in total I figured gain is ~42db, which is about what I'm looking for. And, the 4.7k load resistor lets me try a low-value RIAA network. Gary Pimm has used it in his preamp and liked it, so I figured I'd give it a try here.
I used LTspice to mess with the RIAA components and figure out some DC operating points. There's several 6DJ8/ECC88 models floating around, so I tried more than a couple to get some sense of how the design would wander from tube to tube. FWIW, the RIAA looks to be pretty solid, and the resistor values come right off of Mouser's IRC catalog page.
I tried to keep the bias at >1.7V to stay away from any possibility of drawing grid current and mucking up the RIAA. Consequently, the 6DJ8 are running at around 10mA.
I'm planning on running both channels off of a single, well-regulated B+. I'm hoping the LC (one for each channel) will provide sufficient PSRR/decoupling for the cascode stage, and the mu-follower should have enough PSRR to hold its own. Or am I off-base here?
The final coupling capacitor is a little on the large size, but the next stage has a 10k input impedence. As near as I can tell, the mu-follower will be okay with the 10k impedence.
Any feedback for me?
Regards,
John
I've built a couple of projects from scratch, and for 2010 I'd like to try my hand at a phono preamp. Since it's my first go, I'd appreciate it if I could get some comments, critques, "yeah, but"'s, or what have you. Here's my first shot at drawing something up:
An externally hosted image should be here but it was not working when we last tested it.
A couple comments on the design. The gain in the first stage is a little low for what you might expect out of a cascode but in total I figured gain is ~42db, which is about what I'm looking for. And, the 4.7k load resistor lets me try a low-value RIAA network. Gary Pimm has used it in his preamp and liked it, so I figured I'd give it a try here.
I used LTspice to mess with the RIAA components and figure out some DC operating points. There's several 6DJ8/ECC88 models floating around, so I tried more than a couple to get some sense of how the design would wander from tube to tube. FWIW, the RIAA looks to be pretty solid, and the resistor values come right off of Mouser's IRC catalog page.
I tried to keep the bias at >1.7V to stay away from any possibility of drawing grid current and mucking up the RIAA. Consequently, the 6DJ8 are running at around 10mA.
I'm planning on running both channels off of a single, well-regulated B+. I'm hoping the LC (one for each channel) will provide sufficient PSRR/decoupling for the cascode stage, and the mu-follower should have enough PSRR to hold its own. Or am I off-base here?
The final coupling capacitor is a little on the large size, but the next stage has a 10k input impedence. As near as I can tell, the mu-follower will be okay with the 10k impedence.
Any feedback for me?
Regards,
John
Hi All,
I'm assuming the deafening silence equates to a collective "Meh", so here's another take -- hybrid cascode input stage and a GPimm CCS-loaded output stage, using the mu-follower 'out':
FWIW, I messed around in LTspice to see what kind of gain I could expect with the 4.7k anode resistor, and it's spitting out 47dB for the preamp in total. Is that believable, or will gain vary considerably depending on the specific examples I acquire? I don't have any experience with the 2SK170 or for that matter much SS in general, so if anyone has any helpful hints/ construction tips, I'm all ears.
Better/worse than my previous schematic? Decent probability of building something worthwhile?
Regards,
John
I'm assuming the deafening silence equates to a collective "Meh", so here's another take -- hybrid cascode input stage and a GPimm CCS-loaded output stage, using the mu-follower 'out':
An externally hosted image should be here but it was not working when we last tested it.
FWIW, I messed around in LTspice to see what kind of gain I could expect with the 4.7k anode resistor, and it's spitting out 47dB for the preamp in total. Is that believable, or will gain vary considerably depending on the specific examples I acquire? I don't have any experience with the 2SK170 or for that matter much SS in general, so if anyone has any helpful hints/ construction tips, I'm all ears.
Better/worse than my previous schematic? Decent probability of building something worthwhile?
Regards,
John
John,
1/ 4k7 is very low value for a tube anode load R, more likelyto be seen in a ss amp. We use 25k and even that's low compared to most others. This iwhy youhave such low gain. Set the anode v of your lower tube to around 60V, and the anode of the upper tube to around 130V. Do this by changing the cathode R.
2/ The hybrid will give you even more gain, but again 4k7 is WAY too low an anode load R. Try 25k and in this hybrid's case, the grid of the tube should be at around+7V and it's anode around 70V. With a 260V B+, this will allow you a lot of running current, hence good gain and low noise. This two stage design should be able tohandle MCs with no problems.
3/ Your RIAA network is very low impedance - I'd be looking at (at least) 10x higher impedance there. I.E 10x the R values and 1/10 x the cap values.
This will help put the gain up as well.
4/ The 2SK170 doesn't change gain miuch unit to unit, but matched pairs are advised if you want to do it right.
Regards, Allen
1/ 4k7 is very low value for a tube anode load R, more likelyto be seen in a ss amp. We use 25k and even that's low compared to most others. This iwhy youhave such low gain. Set the anode v of your lower tube to around 60V, and the anode of the upper tube to around 130V. Do this by changing the cathode R.
2/ The hybrid will give you even more gain, but again 4k7 is WAY too low an anode load R. Try 25k and in this hybrid's case, the grid of the tube should be at around+7V and it's anode around 70V. With a 260V B+, this will allow you a lot of running current, hence good gain and low noise. This two stage design should be able tohandle MCs with no problems.
3/ Your RIAA network is very low impedance - I'd be looking at (at least) 10x higher impedance there. I.E 10x the R values and 1/10 x the cap values.
This will help put the gain up as well.
4/ The 2SK170 doesn't change gain miuch unit to unit, but matched pairs are advised if you want to do it right.
Regards, Allen
Thanks a lot for the feedback Allen; I certainly appreciate it.
I recognize that the low anode resistor is responsible for the low gain out of the first stage in either schematic. If you'll indulge a small thought experiment, setting aside the issue of low gain, what are the repurcussions of a small anode resistor on a cascode? Will distortion be remarkably higher for 4.7k vs. 25k resistor?
Also, using a 2SK170BL in the first stage, what are the implications for overload? It looks like for most reasonable values of source resistors, bias works out to be .2-.3V. Is that going to be sufficient for your typical MM cartridge, with up to 5mV?
Regards,
John
I recognize that the low anode resistor is responsible for the low gain out of the first stage in either schematic. If you'll indulge a small thought experiment, setting aside the issue of low gain, what are the repurcussions of a small anode resistor on a cascode? Will distortion be remarkably higher for 4.7k vs. 25k resistor?
Also, using a 2SK170BL in the first stage, what are the implications for overload? It looks like for most reasonable values of source resistors, bias works out to be .2-.3V. Is that going to be sufficient for your typical MM cartridge, with up to 5mV?
Regards,
John
A cascode circuit has extremely high output impedasnce, effectively it's a CCS. So whatever value of resistor it drives makes little/no difference to distortion. And from my measuremets, the distortion is extremely low.
A jfet does not run into gate current until the gate goes +0.7V positive. So with 0.2 to 0.3V neg already - you have NO worries about overload at the input, even from a hot MM cart.
But you shouldn't need a jfet to get enough gain for a MM, with a decent value of anode R you'll be right there all tube.
Regards, Allen
A jfet does not run into gate current until the gate goes +0.7V positive. So with 0.2 to 0.3V neg already - you have NO worries about overload at the input, even from a hot MM cart.
But you shouldn't need a jfet to get enough gain for a MM, with a decent value of anode R you'll be right there all tube.
Regards, Allen
If you mean Vp of 150V and -1.5Vgk with ~20mA this sounds like a reasonable choice for the type.
I'm running mine a little colder with -1.2Vgk (diode bias), 15mA and 100V on the plates with very good results. Note that variations in plate current can be rather large with very small changes in Vgk as well as due to the gm spread from sample to sample so if setting a specific operating point is important to you a CCS removes the uncertainty - in practice I use tube based gyrators as loads in my design and while it is not constant current, the load impedance is high compared to the rp so the gain is very consistent over a wide range of device transconductances.
While I understand what you mean the voltage from plate to cathode, generally Vpk is taken to mean volts peak. I don't recommend you use this term as it is confusing unless you define it clearly in your post. (The problem is the lack of subscripts that would make it clearer.)
More thoughts...
If I'm going to go with a triode-connected D3A in the first-stage, should I go with another D3A in the second stage or should I pick something else based on my target voltage gain? Given the D3A's mu of ~75, I only need a mu of ~22 for the second stage to get me to 44dB, which is about what I was looking for. How about an active-loaded ECC99? Might that work well as the second stage?
With the D3A, g2 is tied to the anode and g3/shield is tied to the cathode, is this correct? And, I'm assuming that all three grids will need stoppers, is that right?
Thanks for the help.
Regards,
John
If I'm going to go with a triode-connected D3A in the first-stage, should I go with another D3A in the second stage or should I pick something else based on my target voltage gain? Given the D3A's mu of ~75, I only need a mu of ~22 for the second stage to get me to 44dB, which is about what I was looking for. How about an active-loaded ECC99? Might that work well as the second stage?
With the D3A, g2 is tied to the anode and g3/shield is tied to the cathode, is this correct? And, I'm assuming that all three grids will need stoppers, is that right?
Thanks for the help.
Regards,
John
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