Capacitive Loading Imposed by the Preamp

[RCAguy]

louie3/Louis & ecospring/Reese & Turbon (you came up having linch with Richard) - just wondering how you're doing with C-loading?  And to point you to the conversation "Tuning..." at http://www.lencoheaven.net/forum/index.php?topic=18231.msg257851#msg257851 and paper on the subject at http://www.filmaker.com/papers/pap_RM-Phono%20C-load%20&%20balance.pdf

[Carlos5302]

This topic started with:

I'm starting this thread in the cartridge subject because loading is so essential to the sound of a cartridge. If we don't apply the correct resistive and capacitive loading to a cartridge, we really don't know how it sounds and what it's capable of.

I was triggered by the word essential, so continued further exploring the subject. Discovered in LH the topic Tuning phonograph reproduction...capacitive loading & channel balancing, which contains a link to the white paper http://www.filmaker.com/papers/pap_RM-Phono%20C-load%20&%20balance.pdf . I own an AT-95E cartridge which requires a load capacitance of 100-200pF, compared to my Shure and Ortofon cartridges requiring +- 400pF load capacitance. I measured tone arm and RCA cable capacitance of one of my TTs and had to face that the AT-95E capacitance load was probably out of spec. So, I decided to build the simple impedance bridge described in the paper. This small project is now finished and I wanted to see for any other LH menbers' experience with that impedance bridge. Unfortunately, there is very few feedback regarding this bridge in LH. Anyone who has unpublished experience who wants to share it?

Karel

P.S.: Start a new topic on this impedance bridge?

[timelog]

For AT95 "recommended load capacitance" is 100-200pF. "Recommended load capacitance" is sum of 1) tonearm capacitance + 2) RCA cable capacitance + 3) input capacitance of phono amp. This leads easily situation where tonearm and RCA cable is already together near 200pF and so input capacitance of phono amp should be set near 0 pF and this is only possible if input capacitor value of phono input can be controlled with switching options. I have tested AT95 and it sounds better when used around 200 pf load, which was done switching phono amps capacitor load off (~0) as tonearm and RCA cable load was around 200pF.

Typical phono input of integrated amp is around 100-200pf and with tonearm and RCA cable capacitance total  load seen by pickup goes easily over 400pf ( depends mainly on cables used ).

[Carlos5302]

Hello timelog, can you share how the listed capacitance values were measured or where the data is coming from?

Ref.:

and this is only possible if input capacitor value of phono input can be controlled with switching options

Another option is to shorten the DIN/RCA cable or to select one with lower capacitance per meter, see measurements below.

Measurements:
Units under test: Lenco L75 turntable, JVC JL-A15 turntable cable, Luxman LV-111 amplifier
Test equipement:
- DIY M8 LC Digital Inductance Capacitance Meter Kit (SKU226140)  (https://www.banggood.com/DIY-M8-LC-Digital-Inductance-Capacitance-Meter-Kit-p-976437.html)
- DIY Simple Impedance Bridge from BAS Speaker of april 1977 page 23 (http://www.bostonaudiosociety.org/pdf/bass/BASS-05-07-7704.pdf)

Measurement results:

• Tone arm:
  Lenco L75 tone arm, no headshell installed, measured with DIY M8 LC kit. Cable length: 47cm, capacitance: ~75pF (1.6pF/cm).
• Cables:
  
  • Lenco original DIN/RCA cable, measured with DIY M8 LC kit. Cable length 140cm, capacitance: ~290pF (2pF/cm).
  • Scrap cable from a JVC JL-A15 turntable, measured with DIY M8 LC kit. Cable length: 145cm, capacitance: ~80pF (0.55pF/cm).
• Amplifier's phono input impedance:
  Amplifier switched on! Resistance was within the range 47..50KOhm, capacitance 25..50pF. I wasn't able to narrow down the capacitance range as the setting of the capacitance on the bridge is coarse and very sensitive to slight deviations of the resistance setting.

Conclusion: I removed the original Lenco cable in favor of the scrap JVC one, as the total capacitance is now 75 + 80 + 25..50 = 180..205pF, which should be ok for the AT-95E.

Karel

Edit history:
20180222: strike through of erroneous data after double checking the preamp.
20180226: see Reply #65

[timelog]

Hello timelog, can you share how the listed capacitance values were measured or where the data is coming from?

I have earlier made some cable and tonearm capacitance measurements (with cheap voltimeter with C measurement input ) and given values just approximations from earlier experiments. I think your measurements where correct as good RCA cable is about 80 pF/m according cable specs.

In LH the topic "Tuning phonograph reproduction...capacitive loading & channel balancing" was information how to implement capacitance switching options and I have my own version of phonoamp circuit where all input capacitors can be switched off. In typical phono input of preamp/integrated amp there is rarely swithing options and if you look repair manual schematics of phono section there is typically 100 - 200 pF capacitor installed as default. That used default capacitors can be in some cases be removed (need soldering) and even replaced with own switching options added to circuit.

Your phono input capacitance measurement value (25..50pF ) is bit low but can be right value. Try to check Amplifier's phono input capacitance from repairmanual circuit layout components.

[Carlos5302]

I measured the amplifier's phono input impedance again with the simple impedance bridge.



When the amplifier is not powered the capacitance measured equals 230pF, when powered on it equals 200pF.
With the JVC JL-A15 scrap cable this results in a total capacitance of 75 + 80 + 200 = 355pF, which is out of the specs for the AT-95E.
Taking into account the quote from timelog:

I have tested AT95 and it sounds better when used around 200 pf load, which was done switching phono amps capacitor load off (~0) as tonearm and RCA cable load was around 200pF.

next step is to have a closer look at the schematics of the LV-111 phono input and re-read the threads Capacitive Loading Imposed by the Preamp and "Tuning phonograph reproduction...capacitive loading & channel balancing".

Karel

[timelog]

Just checked that in Lenco that is in my use gives 230pF/230pF +-5pF (Arm without headshell + ~1m RCA cable). In second arm that is not in use values were 230pF/260pF  +-5pF (Arm without headshell + ~1m RCA cable). Tonearm is connected to RCA cable with applying PC audiocard connector in both cases.

80' simple Pioneer turntable with 1m RCA cable gives ~160pf (RCA cable + Tonearm with headshell without pickup).
DJ player with 1,5m RCA cable gives ~ 190pF (+-10pF) (RCA cable + Tonearm with headshell without pickup)

[martin_bell]

Recently brought a Cambridge Audio 640P to put into a secondary system.

Using a Audio Technica AT-150MLx which recommends 100-200pf. Stock the 640P has 220pf load and the Technics arm around 120pf, what i did is remove the input caps totally and it sounds fantastic. I'll have to tweak this once the SME arm is re-wired as I think that wire has a lower capacitance compared to the Technics.

With the 640P stock it sounded too harsh in the highs followed by a big drop, with the capacitance in the correct zone the harshness is gone and replaced by a nice even top end that's extended.

[Kent T]

Reese - Looks like you'll have 6 selections of capacitance with that 2-pole (2-channel) rotary switch.  Glad to see the cast aluminum box for shielding - and it looks like insulated jacks that can ultimately be grounded through a single path to the box's ground lug.  For the benefit of others who might rig something similar, either remove or take into account the capacitance inside the following preamp, as well as the preceeding phono cable - these are constant, possibly already too large, and will add to the value selected by the box.  You said you have assorted of caps on order, but remember that in a pinch, you can make many values from the 100pF you have, including by series and series-parallel, the 50pF and 150pF values in my 3-position selector at http://www.lencoheaven.net/forum/index.php?topic=14975.30

To louie3's point, do you, Richard/Gene/KentT or anyone else, know what the popularity/acceptance has been of the moving coil cart v MM/MI?  While the rather small audiophile community might lean toward MC, my sense is that broadcasters and other mere mortals went the MM/MI route.  Even the Library of Congress and other restorers!  If so, why would manufacturers favor a MC approach to (no) capacitive loading?  Did the industry "decide" that all C should be in the phono-to-preamp interconnects?  Or at some point did they think vinyl was too small a market for concern?

The Moving Coil has been accepted highly by audiophiles. But not so much for broadcast users in the USA, mere mortals, those who need replaceable styli, those who need multiple styli for their record playback needs, and those on budgets, not to mentioned those who need more ruggedness. Normally the capacitance or C is in the phono stage in use (for many years the built in MM in preamplifiers, and in receivers most often used). A major fact which affected MC's very low acceptance in Broadcast applications is their delicate nature, and non user replaceable styli, given the punishment most commercial DJ operators give styli.

[flood2]

The Moving Coil has been accepted highly by audiophiles. But not so much for broadcast users in the USA, mere mortals, those who need replaceable styli, those who need multiple styli for their record playback needs, and those on budgets, not to mentioned those who need more ruggedness. Normally the capacitance or C is in the phono stage in use (for many years the built in MM in preamplifiers, and in receivers most often used). A major fact which affected MC's very low acceptance in Broadcast applications is their delicate nature, and non user replaceable styli, given the punishment most commercial DJ operators give styli.

All excellent points! One other reason why MC would not be as applicable for Broadcast and DJ work was that MC cartridges tended to have inferior tracking ability compared to the best MM designs which would be a big issue for the increasingly hot cuts of 45 singles and 12" singles. Excellent MCs like the DL304 would be noticeable exceptions, but the superior tracking ability of the Stanton and Shure MMs of the day and the economy of user-replaceable styli would make them the obvious choice for professional applications. Secondly, the wide bandwidth promised by the low inductance of low output MCs would be lost completely given the 15kHz bandwidth limit of standard FM broadcast anyway, thus rendering the additional cost of MC completely unwarranted.

[flood2]

Recently brought a Cambridge Audio 640P to put into a secondary system.

Using a Audio Technica AT-150MLx which recommends 100-200pf. Stock the 640P has 220pf load and the Technics arm around 120pf, what i did is remove the input caps totally and it sounds fantastic. I'll have to tweak this once the SME arm is re-wired as I think that wire has a lower capacitance compared to the Technics.

With the 640P stock it sounded too harsh in the highs followed by a big drop, with the capacitance in the correct zone the harshness is gone and replaced by a nice even top end that's extended.

The 120pF load you currently have already puts the electrical resonance well beyond the audible range at 24.7kHz (assuming the default resistive load of 47k) therefore you are unlikely to hear any obvious difference in the audible range due to the capacitive load change if your SME arm has an even lower total capacitive load. The differences you hear will largely be down to the differing mechanical resonances due to interactions with the arm components and the cartridge suspension.

As the mechanical resonance characteristics also depend to an extent on the interaction with the arm components (counterweight mass and position, headshell rigidity and various other factors) which themselves contribute to resonances in the audible range, audible changes in the tonal balance are unlikely to be affected signficantly by changes to the capacitive load given that the effect of the electrical loading is well above the audible range. If you hear a signficant change, it is likely that the output around 8 to 12kHz is the region being affected (which is why the treble control is typically tuned around that frequency) which would be largely untouched by the electrical resonance. 

It is noteworthy that AT MMs all seem to be "voiced" to have a slight emphasis of about 1dB or so between 10 to 12kHz which is why one often reads "complaints" about the AT440ML (for example) being too bright. I have several AT MMs (100E, 120E, 440ML, 540ML, 150MLx) and they all measure the same white noise response irrespective of capacitive load (within the recommended range).

[martin_bell]

The 120pF load you currently have already puts the electrical resonance well beyond the audible range at 24.7kHz (assuming the default resistive load of 47k) therefore you are unlikely to hear any obvious difference in the audible range due to the capacitive load change if your SME arm has an even lower total capacitive load. The differences you hear will largely be down to the differing mechanical resonances due to interactions with the arm components and the cartridge suspension.

As the mechanical resonance characteristics also depend to an extent on the interaction with the arm components (counterweight mass and position, headshell rigidity and various other factors) which themselves contribute to resonances in the audible range, audible changes in the tonal balance are unlikely to be affected signficantly by changes to the capacitive load given that the effect of the electrical loading is well above the audible range. If you hear a signficant change, it is likely that the output around 8 to 12kHz is the region being affected (which is why the treble control is typically tuned around that frequency) which would be largely untouched by the electrical resonance.  

It is noteworthy that AT MMs all seem to be "voiced" to have a slight emphasis of about 1dB or so between 10 to 12kHz which is why one often reads "complaints" about the AT440ML (for example) being too bright. I have several AT MMs (100E, 120E, 440ML, 540ML, 150MLx) and they all measure the same white noise response irrespective of capacitive load (within the recommended range).

Very interesting.
Re voicing have a look at this review, this is the replacement of the 440MLa and appears to measures quite flat with the reviewer noting that they seemed to of removed that little bump.
The same stylus is also used on the more expensive VM-740ML which according to AT is the equivalent of the 150MLx and said this is the replacement stylus I should buy.
Funny thing is according to that reviewer that stylus on the 740ML body the emphasis returns! So it seems the more expensive models are indeed voiced.

https://hometheaterhifi.com/reviews/vinyl/turntable-accessories/audio-technica-vm540ml-phono-cartridge-review/

I'm actually contemplating a OM-40 stylus for my OM-10 when the AT-150MLx needs replacement but I'm not sure, I'm mostly happy with the sound of the AT but most of all I love the low distortion and I haven't had great experience with Ortofon in the past re tracking but they were all elliptical models but still overbuilt cantilever's. Although the OM properly loaded produces what I feel is a good balanced sound
  

[martin_bell]

Oh also what test records do you use that has white noise? I mainly use an old Shure test record to set anti-skate and determine rough resonant frequency but would be nice if it had some more useful noises on there

[flood2]

Oh also what test records do you use that has white noise? I mainly use an old Shure test record to set anti-skate and determine rough resonant frequency but would be nice if it had some more useful noises on there

Two options - one is to correct the pink noise test tone response post recording from any of the existing test discs (like HFNRR, Ultimate Test LP, Feickert Adjust+ etc) or the original Ortofon TC002 test disc which has a white noise (RIAA eq) test tone for vertical and lateral modulation to test phase. There is also a linear white noise test for SNR if you want to use digital EQ
TC002 is still available from Discogs: https://www.discogs.com/sell/release/3155906?ev=rb

[martin_bell]

Two options - one is to correct the pink noise test tone response post recording from any of the existing test discs (like HFNRR, Ultimate Test LP, Feickert Adjust+ etc) or the original Ortofon TC002 test disc which has a white noise (RIAA eq) test tone for vertical and lateral modulation to test phase. There is also a linear white noise test for SNR if you want to use digital EQ
TC002 is still available from Discogs: https://www.discogs.com/sell/release/3155906?ev=rb

That's fantastic, thank you.