L-pad instead of H-pad??

[limey222]

I'm following some written instructions as i slowly modify two Roberts/Akai mono block amps to become mic preamps. I've been told to convert the unbalanced phone jack output to a balanced XLR output by disconnecting the original phone jack wiring, installing a 500 ohm resistor on the signal output side, followed by a 1:1 ratio output transformer such as a Jensen unit. I'm told that the signal will still be too large at this point and that i will need to install an H-pad as most modern day mixers can't handle such a high voltage signal. My question is why can't i just install something like a 10K Alps Blue velvet poteniometer on the output side immediately after the transformer to control output level? I can buy them, I can't buy an H-pad.

[HollowState]

If you are converting a single ended output to a balanced output, then you must use a balanced attenuator like an H pad (also known as a balanced T) where an L pad is a single ended device. An H pad maintains a constant impedance on both the input and output side. An L pad is constant only in one direction. (input)

When you go to professional balanced circuits you must do things that comply with the system requirements. Trying to use a dual L pad would lower the output but you could easily experience frequency aberrations from the mismatch.

One alternative would be to wire in a fixed H style attenuator using resistors. Then wire in a DPDT toggle switch to jump them if/when not needed.

A second alternative would be to somehow lower the gain before the transformer.

Victor

[Steve Eddy]

Do you know how much you'll need to reduce the signal level?

You could use a 150 x 4 output transformer wired 2:1 which would reduce the signal level by half (-6dB).

se

[limey222]

Originally posted by HollowState
If you are converting a single ended output to a balanced output, then you must use a balanced attenuator like an H pad (also known as a balanced T) where an L pad is a single ended device. An H pad maintains a constant impedance on both the input and output side. An L pad is constant only in one direction. (input)

When you go to professional balanced circuits you must do things that comply with the system requirements. Trying to use a dual L pad would lower the output but you could easily experience frequency aberrations from the mismatch.

One alternative would be to wire in a fixed H style attenuator using resistors. Then wire in a DPDT toggle switch to jump them if/when not needed.

A second alternative would be to somehow lower the gain before the transformer.

Victor

Could I use an L-pad prior to the output transformer?

r

[Steve Eddy]

Originally posted by limey222
Could I use an L-pad prior to the output transformer?

Depends.

What sort of load can the output of the amp drive? Can it handle a 600 ohm load? If so, you could use a 600 ohm T attenuator. Otherwise if it wants to see a high impedance load, then that won't work so well as the output impedance will be rather too high for the output transformer.

se

[EC8010]

Can I say this once and for all? 600 Ohms went out with the Ark! Professional audio expects to see low source and high destination impedances. Microphones are typically 200 Ohm source, microphone amplifiers are a maximum of 13.6k (2 x 6k8 for the phantom power) input.

Balance is crucial. If you have to use attenuators, they must be balanced T or pi.

[Steve Eddy]

Originally posted by EC8010
Can I say this once and for all? 600 Ohms went out with the Ark! Professional audio expects to see low source and high destination impedances. Microphones are typically 200 Ohm source, microphone amplifiers are a maximum of 13.6k (2 x 6k8 for the phantom power) input.

True enough.

But the question was if he could put the attenuator ahead of the output transformer. And in that position, there are two things to consider, the load seen by the output stage, and the source impedance seen by the output transformer.

Since the output transformer doesn't want to be driven by much more than about 600 ohms, then the output would need to be able to drive about 600 ohms.

But that would only be for a T or H pad which have the same input and output impedances. I just noticed he said L pad, and in that case, he could use a shunt resistor of about 600 ohms and adjust the value of the series resistor until he achieves sufficient attenuation.

Balance is crucial. If you have to use attenuators, they must be balanced T or pi.

They wouldn't need to be balanced if they're going to be placed between his existing unbalanced output and the output transformer.

se

[limey222]

Originally posted by Steve Eddy


True enough.

But the question was if he could put the attenuator ahead of the output transformer. And in that position, there are two things to consider, the load seen by the output stage, and the source impedance seen by the output transformer.

Since the output transformer doesn't want to be driven by much more than about 600 ohms, then the output would need to be able to drive about 600 ohms.

But that would only be for a T or H pad which have the same input and output impedances. I just noticed he said L pad, and in that case, he could use a shunt resistor of about 600 ohms and adjust the value of the series resistor until he achieves sufficient attenuation.

They wouldn't need to be balanced if they're going to be placed between his existing unbalanced output and the output transformer.

se

OK, now i might be getting somewhere, in my original post i had already stated that I had been instructed to install a 500 Ohm resistor inseries between the output stage and the output transformer. I assume that the "shunt" resistor mentioned is in addition to that? Is that installed in a different location or is it one of the same thing. BTW, I'm going to use a Jensen JT-11-DMCF which should drive 600 ohm loads to level up to +22dbu @20HZ according to the manufacturer's specs. I assume that I was told to use the 500 ohm resistor to knock-down the signal level going into the transformer since even at 1:1 the transformer output will be too high for me to effectively utilize for anything in my studio except to drive speakers, it's certainly too high to drive a mixer board input or a studio quality sound card.
What I want to do here is to be able to adjust how hard I drive to amplification stage of the mono blocks to give me varying amounts of tonal control over the signal, (read small amounts of pleasant tube distortion) but then also control in a variable manner, the output being presented to the mixer so that I don't overload the mixer input. It seems to me that using a fixed manner to know down the signal level is a bit bit hit and miss. Is there no variable method to control output available?
Please remember that I am new to all this so go slowly and be as specific as you can...thanks

[Steve Eddy]

Originally posted by limey222
OK, now i might be getting somewhere, in my original post i had already stated that I had been instructed to install a 500 Ohm resistor inseries between the output stage and the output transformer. I assume that the "shunt" resistor mentioned is in addition to that?

Essentially, yes.

Is that installed in a different location or is it one of the same thing.

Basically it would be two resistors tied together, with the 600 ohm across the transformer primary and the series resistor tied to the top of the primary and the other end fed from the output of the amplifier.

BTW, I'm going to use a Jensen JT-11-DMCF which should drive 600 ohm loads to level up to +22dbu @20HZ according to the manufacturer's specs.

Have you purchased the Jensen yet? If not, as much as I love the folks at Jensen, I'd recommend going with CineMag instead. Their transformers are every bit as good as the Jensens but cost significantly less.

I assume that I was told to use the 500 ohm resistor to knock-down the signal level going into the transformer since even at 1:1 the transformer output will be too high for me to effectively utilize for anything in my studio except to drive speakers, it's certainly too high to drive a mixer board input or a studio quality sound card.

Not sure why you were told to use the 500 ohm resistor. It may have been for the reason you state, or the person may have been under the mistaken impression that the transformer needs to be driven with a 600 ohm source impedance.

By the way, you say these are "mono block amps." When I hear "mono block amps" I think power amplifiers. Are these in fact power amplifiers?

What would be most helpful here would be to know exactly what you have, what sort of voltage gain they have, and ideally the schematics.

There may be other, better solutions to what you want to achieve here.

What I want to do here is to be able to adjust how hard I drive to amplification stage of the mono blocks to give me varying amounts of tonal control over the signal, (read small amounts of pleasant tube distortion) but then also control in a variable manner, the output being presented to the mixer so that I don't overload the mixer input. It seems to me that using a fixed manner to know down the signal level is a bit bit hit and miss. Is there no variable method to control output available?

Well, the series resistor could be a potentiometer which would allow the attenuation to be variable. But before going too far down that road, let's find out exactly what you've got.

Please remember that I am new to all this so go slowly and be as specific as you can...thanks

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se

[limey222]

I have no circuit diagrams just the text of the original ebay listing copied below:

"Offered at auction are a matched pair of classic Roberts model 770X monoblock all tube combination microphone preamps / power amplifiers in superb condition. NOTE: These are not the usual pair of stock units requiring extensive service and modification, but instead are fully serviced, professionally customized and blueprinted examples of these popular amps. As a retired old school recording studio service/calibration tech, with decades of experience in designing custom tube preamplifiers for demanding studio applications, I fortunately still have provision for the (fairly extensive) setup and calibration procedures required for proper reconfiguration and bench testing of these models. For those not familiar with these early mini rack units, first used in the excellent sounding reel to reel recorders marketed both as Akai and Roberts, and using the same basic circuitry in Models such as the 990, 770 X, M-8, etc., they have long been very popular for use as stand alone Mic preamps in both live and recording venues. The original circuit topography, and superior build quality, allowing both microphone, as well as normal line inputs, making them ideal for tweaking and modification.
First the original 6267 triode/pentode tube and related circuitry (used for the massive equalization and current requirements in the NAB tape recording and playback head circuits of the recorders) has been removed. This section, as well as the related 6AL5 tube used in the erase and bias section (also removed) effectively have nothing to do with the use of these units as designated, only adding noise and sonic degradation if left installed. What remains is the essence of classic Single Ended tube elegance, a huge power supply transformer, supplying well filtered B+ voltage and current reserves, tube rectified (using the proven 6X4 rectifier) to the remaining SE operated dual triode high gain and driver stages for the single 6BQ5/ EL-84 SE output pentodes, operated in pure class A tetrode mode. All superfluous wiring, cabling, etc originally installed for dedicated open reel interconnection, again has been removed, with only the front panel input and output jacks (standard 1/4 inch phone type) and separate, polarity reversable standard line cords (allowing true monoblock configuration) remaining, for easy, user friendly operation. All original power supply and active circuit components have been separately tested for value and tolerance with any required replacements done using original Akai Japanese exact equivalents. All critical coupling and bypass capacitors have been replaced using bench tested and VERY closely matched NOS high voltage American Sprague Black Beauty types, proven in this circuitry for their low noise, and low DC leakage capability. The single remaining variable potentiometer, (the critical volume/gain pot for both units) has been replaced with quality early generation Alps, again, hand measured for value, low noise, and tracking linearity. These are exceptionally quiet circuits in operation, with no prevalent PSU 60HZ, 120Hz, or higher order components, and only extremely low level, wide band (pink noise) measurable, but totally non intrusive in normal operation. This residual noise floor (common resistor and tube micro hiss) is much lower than is normally found in even many high end studio and home pre amps. The large blackface upgraded lighted analog metering, has been calibrated, with superb accuracy, channel tracking (better than 1/4 Db full scale) and currently showing 0 Db at the very onset of soft natural tube compression, and +2 Db at threshold of soft (and uniform) clipping. No Tube amplifier is any better than the components used in it's design. The phenomenal output transformers supplied in these early models is superior in performance in every measurable and audible parameter. Power output waveforms including demanding square wave analysis, reveal oscilloscope traces virtually unseen in lesser SE designs, with superb full bandwidth linearity, no ringing or overshoot even when driven into soft clipping, and transient response rivaling the best of P/P circuits. These are very good sounding amps, capable of all of the sonic virtues associated with Single Ended mythos, and seemingly none of the (often not spoken of) shortcomings. The properly biased Class A outputs, (Unbypassed Cathode bias) allow the supplied (low hour, matched used USA GE 6BQ5/ EL- 84s to couple through the 8 ohm rated transformers in such a way that appears uncanny in my experience. Square wave output wave forms are virtually uniform in linearity, bandwidth and harmonic detail from below 4 ohm dummy loads to open circuit conditions, with electronic and thermal stability unchanged regardless of load impedance or reactance. The 12AD7/12AX7 class of dual triodes specified for use in these circuits allows for a vast range of tube substitution tweaking. In my experience the finest design for use as HiFi amps, (with equally low noise, and bandwidth capability in high gain mic use, is the venerable British Mullard ECC83/ 12AX7, and as such these are supplied with low hour used and perfectly matched examples with intact silkscreening, ready to use. The supplied rectifiers are a matched pair of the proven Sylvania USA black plate 6X4. I will try to sum up this admittedly over wrought description: The multi position slide switch used originally to switch between record and play modes for the recorder is securely bolted into the correct fixed position. The cut type tone control is of course, no longer active and the original knobs are configured in this way. The first (left knob) controls volume/gain and the right set up operate the two position control, orig. used for speed equalization switching. As currently modified, this switch now allows use in one position for absolute flat response, with a second option giving a low order rise in high frequency response (begining at just below 11 Khz to a maximum peak of +4 Db at 17 Khz). Experience has shown over the years, that this relatively subtle curve is ideal for touch up of mic response from a large number of vintage designs, allowing just the right increase for miking higher detail for Piano, cymbal, etc. harmonics, from less than ideal microphone sources. This slight (but distinctly) noticable response change, also serves very well to compensate for the often time, too soft high end response of vintage full range and tweeter loudspeaker designs, the use of which is mandated by the efficiency requirements of the 4 watt RMS + output of these amps.
I have purposely left the input and output circuitry (and operating level) options for mic pre use, up to the potential high bidder. The Mic inputs are currently set up for use only with high Z unbalanced mics (with standard 1/4" jacks). The conversion to balanced input is of course, easily facilitated by use of any common Low Z (600 ohm) balanced to grid input transformer (either externally or internally mounted) and the required XLR or TRS jack for connection. Traditionally the line outputs for these are optimized by use of the power output section (again 1/4" phone jack) for interface. The superb wide bandwidth linearity and stability into practically any load impedance, makes this configuration possible.This allows for the ideal current source, low impedance output capability to be used as is (for unbalanced input to vintage line level boards and mixers, etc.) and attenuated using the receiving units stepped or variable pad or input operating level attenuator. If lower source voltage is required only the addition of a simple fixed or variable voltage divider network is required to preset output and (purely resistive source impedance) to any standard operating level from extremes such as -20 db to classic +10 db levels. For balanced output use, again, a matching transformer and jacks are required, but the low output impedance provided, allows for use of the much less costly (compared to quality plate to balanced line) low Z to balanced low Z designs (600 to bal. 600 works nicely). These narrow faceplates allow for use of the remaining EIA rack width (when used with rack extensions) as excellent patch bay panels for installation of the desired components and jacks for custom setups.
For the recording engineer, home HiFi purest, or anyone desiring the versatility, and audio capability of these lovely little monoblocks, without the extravagant price demanded for the modified Designer capped Beautique versions (which are probably quite nice), but perhaps not worth the many hundreds each, often charged, here may well be an excellent opportunity. I am selling these Roberts units as a pair with NO RESERVE. "

So yes, they are mono block amps and yes, i do want to finish off the conversion and make the best of it since I have all the necessay components.

So should I use a 600 ohm shunt resistor across the primary of the output transformer and then use the Alps Blue velvet 100K pot as my series resistor between the amp output and the top of the primary? If not what fixed value should I make the series resistor ?
If I decided not to make the output balanced, and retained the present unbalanced setup and just used a series resistor and the Alps pot (L-pad) to knock down the signal level, would I likely still have a viable mic pre as long as i kept the output line short to the mixer?