|'); } // End -->|
I'm hoping some of you knowledgeable folks can advise me on a phono stage. I am using a Classe 5 ONLY for the MM RIAA but do not have enough gain (35db), and would like to spare up the room as well.
There are so many units out there it would be esspesially nice to get some feedback from people who've improved from classe phono.
Currently using a Oracle/alphason/dynavector 17 mk2 front end. This is transformer amplified 37db and into my Classe 5, Then into my DIY S&B TX103 silver pre.
I know the lack of gain is due to the S&B only having 6db in boost mode, but after hearing this thing I am NOT willing to replace it.
I would like to not only replace the RIAA with a higher gain unit, but also improve it as well ;)
Ideally it would be great to keep the budget around $1500, but I do have squirming room!
Any advice is welcome and appreciated!
The Trichord Dinoand Diablo phono stages give you up to 74db of gain for MC sources. You have options for power supply. If you have the funds, go for the "never connected" power supply for either one.
I don't see the problem or rather can't see a possible solution with a change of the phonostage. If I understand you correctly you are using a 37dB step-up into a 35dB RIAA section of the Classe preamp which goes out via the tape out to a 6dB gain linestage. And you don't have enough gain?
I may have expressed the question incorrectly. I plan to keep the Trans. unit and am looking for a MM/phono with higher gain and better quality. A tough nut, I know. That's why I'm inquiring.
This is available in a low gain 75dB or high gain 90dB model (perhaps high gain and higher gain would be more accurate).
The unit you refer to is a current amp design which has a fixed input impedance of virtually 0 (ZERO) ohms. This means the utput voltage of the cartrdge used is reduced to a fraction of the normal output so the amplification has to be 75 r 90 dB just to get a useable output. Check out the various reviews and you will see even with the 90dB version the effective output to the linestage is barely adequate. I am not against this kind of amplifier but one must understand what's going on here.
A typical MC cartridge loaded at 1 ohm (and we're talking here of virtually 0 ohms) has an output 1/10th that when loaded at the DC resistance, more or less and about 6% of the output compared to a load at ten times the DC resistance of the cartridge.
So the effective gain, amplification is not 90dB but only 70dB at 1/10th the cartridge output (compared to a DC resistance laod) and only 65dB compared to a load of 10x the DC resistance. And that is the 90dB version the 75dB version is much "worse".
So as you can see the gain in dB cannot be taken at face value by such a product.
Nevertheless the total 72dB gain he now has, assuming I did not misunderstand, would only be achieved with one or two very expensive products and I am skeptical about the quality of that high an amplification. The high feedback etc. could be very detrimental to the sound.
So how much of this weaker phonocube "gain" would one need for a 0.4mV output cart? (I noticed there is a kit to make a phonoclone and am curious about it, but not sure now about how it might be set up.)
Unfortunately they are not frank enough to tell you the whole story.
A cartridge with "your" 0.4mV output but with 40 ohm source impedance will have a significantly lower output compared to one with 0.4mV output and only 4 ohm source impedance when the load impedance is virtually zero.
The first cartridge has a short circuit output current of 10 microamps, the second has 100 microamps. A voltage amplifier would see tham as almost the same, a transimpedance amplifier sees the second as having ten times the ouput of the first.
It's a bit like the difference between a garden hose and a fire hose, both operating at the same pressure. If you just measure the pressure, they are the same. If you measure the flow, they are not.
In this analogy a transimpedance amplifer is like a turbine wheel - it doesn't much care about the pressure, it responds to flow rate.
You are right in what you say but it is worth noting that 47 labs have done themselves and others a disservice by talking about the voltage gain of their phonostage. This is a meaningless specification. Because it uses current input and voltage output the correct specification is the transfer function in volts per amp (and therefore numerically equal to a resistance in ohms). In this case we would use volts per milliamp or millivolts per microamp.
I believe the 47 labs units have transfer functions of somewhere about 10 volts per milliamp on the lower setting and about 60 volts per milliamp on the higher setting. All you then need to know is the current output of the cartridge in use (divide the output voltage by the coil resistance, so for the 47 labs Miyabe the output is 150 microamps). This multipled by the transfer function gives the output (around 1.5 volts for the Miyabe).
The lower the DC resistance of the cartridge the less the "impact" will be relative the zero ohm input impedance.
What do you mean by transfer function? How do you know that is with this product and how does it relate to a more traditional voltage amplifier?
I appreciate the input but frankly almost no one can use it in the way you express it. It means nothing.
I'm not sure I can explain it better but I'll try. The first thing you must realise is that the phonocube does not amplify a voltage, it converts a current to a voltage. It is acting as a transimpedance amplifier, the operation of which is pretty standard in instrumentation. As I will explain, there is virtually no voltage present at the input so there is no point talking about voltage gain.
There is however a current present at the input, and since moving coil cartridges operate happily as current sources this works very well. Without boring you with op-amp feedback theory (yet), suffice it to say that output will rise to a level where the current in the feedback loop balances the input current, and the amplifier therefore has a definite ratio of output voltage to input current. Since it is transferring one quantity (current) at the input to another quantity (voltage) at the output this ratio is properly called a transfer function. I calculate that the transfer fuction of the 47 labs unit is around 10 volts per milliamp on the low gain setting.
We can find the closed circuit output current of any moving coil cartridge to a reasonable approximation by considering it as a current source shunted by its own coil resistance. The value of the current source is thus its open circuit output voltage (the output voltage normally specified) divided by the shunt value. This has nothing to do with voltage ratios at the input or any of the guff written by the reviewers, it is simply derived from Kirchoff's Laws.
So we have a specified current and a definite transfer function. If we multiply one by the other, we get a definite output voltage. Simple.
If you want to know how a transimpedance amp works, read on.
The input is directly to the inverting input of an op-amp, with the feedback loop also connected from the output to the inverting input. The non-inverting input is grounded. The function of the loop is to keep the voltage at the inverting input equal to the voltage at the non-inverting input. As I said the non-inverting input is grounded so its voltage is zero and the inverting input is thus also kept as close to zero voltage as the op amp can achieve. This means that whatever current you put in to the input, almost no voltage appears at the input, so it has zero input impedance. The voltage at the input is actually the output voltage divided by the open loop gain of the op amp so the effective input voltage is a function of the amplifier performance. Talking of voltage gain is therefore meaningless.
That certainly sheds a lot of light on the subject. Thanks for showing me the way, and for the url. I'll download gnucap and gwave and play around with them when I have some time. I always thought that the very high "gain" spec for the phonocube was for old very low output MCs like the Ortofon MC20 with output spec of 0.07mV. (How much gain would they need from a voltage amplifier?)
How did you calculate the transfer function? Of course it has something to do with the resistance from the opamp's -ve input to its output (in the link called RF). Is the transfer function simply equal to this resistance?
I simply figured that they had used the same half arsed method of measuring gain that they were recommending to others and applied it to their own cartridge (the Miyabe), worked out the output they should be getting and applied it to the current output of the Miyabe to derive a probable value for the transfer function.
If you use a single step transimpedance amplifier then the transfer function is indeed set by the feedback resistor. This is not usual practice, and is very unlikely for a phono stage. In all cases trying to get very high gain in the first stage is an invitation to parastitic capacitance, so it is usual to make the gain of the first stage sufficient to reduce the noise in the later stages then cascade voltage amplifiers after it. In a phono stage you have to insert the RIAA equalisation components somewhere, so a second stage is needed.
The Audio Research PH 5 lists for $2000. It has 57.5 dB of gain without a transformer, and the load can be changed remotely on the fly. I have a DV 20X, and it works superbly with it. Before I used the Plinius Jarrah and the EAR 834p, modded and tube-rolled, and the PH 5 is in another class; there's really no comparison. But be sure to hear it first, as with all things. There are others in this price range that you might like better, though I didn't find any.
I forgot to ask if your DV 20x is the low or high output model??
Thanks for the advice! I'll find a AR dealer locally but I don't recall ever seeing one. Is this a tube unit?
Dynavector P-75 in PE Mode, especially since you already have the 17 mk2. I've compared both the Stealth and the P-75 at various settings in my system with the DV20-XL.
Just my opinion based on what I own and my listening tastes. Although, it was my friend's Stealth and he had it up for sale the very next day. They're just different. I don't know if one's better than the other. However, the P-75 is better if you factor in price.
Rogue Stealth for $800 and spend the other $700 on LPs or a vacuum RCM.
Try the Dynavector P-75 phono stage, or the Benz Lukaschek.
This post is made possible by the generous support of people like you and our sponsors: