rjp
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Posts posted by rjp
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1 hour ago, NOSValves said:
Actually no they are never 100% out of the circuit they always add a resistive path to ground...
Fair enough. I meant they were 100% out of the signal path.
I wonder, can the additional path to ground make any difference in the sound?
The attenuator Bob supplies is 100K. The VTA board itself has an input impedance of about 270K I believe. So the (full volume) input impedance at the RCA jacks with the stepped attenuator in place must be down to about 75K. The output impedance of an iPhone 6S is 3.2 Ohms, so even 75K seems huge in comparison. A higher resistance attenuator could be used to keep the input impedance up, but that would potentially introduce another more significant problem. Loss of high frequency content due to Miller effect. I assume this played into the 100K choice.
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1 hour ago, wdecho said:
If you only need 1V rms for full output there is no need for pre except maybe with phono. Most any source will drive to full output even computer. If nothing wrong with members amp I think if may just be he is not familiar with quality stepped pot. Most consumer amps pots reach almost max power at 10:30 whereas better equipment take full advantage of control over volume. I like to reach my listening level of sound around 2:00 PM on the potentiometer.
Technically speaking 100% correct. Zero NEED for a preamp. But preamps (good ones, I'm told) add a host of magical musical qualities and gonads (a.k.a., dynamics). I have recently been involved in extensive research and technical discussions on this matter, and though I could uncover no good technical reason for the preamp, I just bought two of them so I can find out for myself if they really make the VTA70 sound better (compared to the 21-step attenuator direct approach).
I purchased a Dynaco PAS 3X from ebay and a VTA SP12 kit from tubes4hifi. I am really looking forward to building the SP12 kit! If I realized I could get the basic board for under 200 bucks I would have skipped the PAS option. Well, it will be fun to try both. I'll keep the one I like and sell the other. They will both be in super amazing condition when I'm done owning them.
So...I intend to be in serious preamp mode shortly
You know what's the dumbest thing? Calling a simple potentiometer or stepped attenuator a "Passive Preamp". Well, actually, that's just a misnomer. The dumbest thing is paying 800 dollars for "really good ones"
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7 hours ago, Emile said:
rjp, thanks! I have the (standard) Bob Latino ST-70, with the 21 step attenuator (and Russian 6550 tubes and caps).
Understand the linear vs log attenuation now - thanks! But ... still a bit puzzled why the 35 Watt ST-70 gives me a lot less volume than my 35 Watt vintage Kenwood. Well, moot point now as my wife is returning from a trip ... meaning won't be checking max sound levels till she leaves again
Cheers, Emile
Ok, so the amp you have is a Bob Latino ST70. Most people call it a VTA70 (Vacuum Tube Audio) to distinguish it from the original Dynaco ST70. This is an amp I am very familiar with. The input level required for full power out is 1V rms. I bench tested mine and was able to get 38W rms per channel before I started to see clipping. This was with both channels driven into 8 Ohms at 1KHz. The input voltage required to achieve this was just slightly over 1V rms (i.e., 1.4V peak).
Any modern input source can easily provide the required voltage swing to get the full 35W rms out of the VTA70 without an active preamp. It could be that the Kenwood is boosting the input signal more so that the overall output is louder but the peaks are clipped. SO it could appear louder but there might be distortion present. Just a guess.
Are you saying that if you set the volume to full on both amps your VTA70 is still not as loud as your Kenwood?
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Emile,
Are you running an original ST70 or a modified version? Where is the attenuator you mention installed?
BOTH a stepped attenuator and ANY volume pot of ANY taper will be effectively out of the circuit at full clockwise rotation (well, unless it's wired up wrong).
What is you input source?
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18 hours ago, Emile said:
Know this is an older thread, but did not want to start a "new" one.
Any of you ST-70 builders ... are you getting enough output from this amp?? Running mine with a pair of Forte's and have to turn it way, way up (3 clicks) from max to get 85-90dB from my speakers
(Compared to maybe 20% on the volume knob of my same wattage Marantz 1060) No clue
Any thoughts are appreciated.
Cheers, Emile
Emile,
I think I know what you are talking about, but after reading some of the interveaning replies I am a little unsure now, so I will just speak about my experience with stepped attenuators.
I had a "regular" volume pot on my VTA70 before I installed a 21 step attenuator. My first reaction was, "Why is there so much less volume now?"
There is not less volume. It has just been redistributed.
The difference was entirely due to the taper of the designs.
The stepped attenuator (at least the most common ones) have steps that are roughly 3dB each. This means that just 2 steps down from full volume is half power. This is known as a logarithmic taper (a.k.a. Audio taper). The potentiometer I was using was a linear taper. So even though the two devices spanned a total resistance of 100K Ohms, they did so in very different ways. Most volume controls on consumer stuff are linear taper, and consequently that's what a lot of us are used to. Most audiophile stuff, however, tends to be logorithmic. WIth a linear taper control the volume initially increases quickly as you turn the knob clockwise, and then slows down. I prefer linear personally.
You can get a stepped attenuator in linear as well if you like the "advantage" of discrete steps rather than continuous adjustment. Same price on ebay. Drop in replacement.
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3 hours ago, mikebse2a3 said:
First hey rjp if you don't want these pictures in your thread I'll remove them so just let me know.
miketn
Thanks for your consideration Mike, but I don't think of it as "my" thread. It is for all of us. Besides, after 11 pages I think there were only a hand full of posts actually discussing the HIII's.
Which, btw, I am liking more and more lately. Ultimately, I decided to return the Monitor Audio Silver 100's and keep the HIIIs.
I may put L-pads in them some day
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picoBells rule!
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I like tone controls as much as I like L-pads. And we all know I like L-pads, right?
I see no problems as long as when they are set to zero I hear absolutely no change in the signal compared to a direct path. I auditioned and ultimately returned a product called miniDSP HD for this reason. I rigged up an A/B switch on the line level inputs and with all EQ functions on the miniDSP set to off I could still hear a definite degradation in the sound quality. I assume this was due to the A/D - D/A stage itself which is essential for any digital tone control product of course. It was subtle but consistent. I performed the same test on a Schiit Loki 4 band EQ and it passed with flying colors. I may have already mentioned this above. I forget what thread I'm on sometimes.
My advice is if you have to have tone controls, get the good Schiit
http://www.schiit.com/products/loki
Speaker placement is effective. Many people roll tubes as a method of tone control as well. All methods have one thing in common: they emphasize some frequencies more than others. It's all "tone control".
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I know you try to help members. Thanks for your contributions. Let's just move on for now and put this behind us.
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Back on Page 3 we had this conversation. I told you I like L-pads.
I even told you I bought the same L-pads as you from Parts Express for just this purpose.
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Do they make analog versions?
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2 minutes ago, Westcoastdrums said:
I have worked in studios and this is the fastest and easiest way to tame something you don't like and is done in seconds by professionals that are responsible for the end product t thay you hear.
Me too. I learned to love the PEQs from working sound for bands. But where can one buy a nice *analog* 2-band PEQ for home use that doesn't look like a guitar stomp box?
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Just now, Westcoastdrums said:
You guys really think the heresy III is too hot anywhere? To each his or her own but these speakers are SMOOTH. I stand by what I say that klipsch should be marketing the heresy III very hard and be giving proper demos of these speakers to get people hooked into the Heritage sound. They are an easy gateway drug. These speakers are INCREDIBLE for their price point.
It depends on the room I suppose. Maybe "too hot" is not the right word. It's just that sometimes I want them to calm down and be mellow for a while
It's hard to describe musical stuff with words.
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Westcoast, perfect!
This is why I like parametric EQ the best. Find the "annoying" frequencies and cut them out. It's hard to do this with a graphic even with 1/3 octave sliders. Give me to good PEQs any day!
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8 hours ago, wdecho said:
I just noticed that you are the OP, original poster of this thread. Why are you complaining on a social media forum about a too hot speaker you have just bought when you are now showing with the above formula and calculations that you have the ability and electronic background to understand why in your opinion that adding a simple out of the box L-pad is not the correct way of attenuating a too hot driver in a pair of new speakers you have just purchased? Why are you not just reverse engineer the crossover installed in your speakers to understand the circuit the engineers at Klipsch deemed as sounding best for their new pair of Heresy speakers. After understanding how the circuit works then use the formulas and electronic calculations to add a few more db's of attenuation and still keep the integrity of the original design. I believe I could and from this post you seem to have a better understanding of electronic formulas and calculations than I do. Or if you do not want to do it this way then design a new crossover to suit your expectations on how a crossover network should be designed.
I am more confused now about your complaining on a social media forum about a new pair of speakers when you are showing you have the ability to correct the problem without any help from anyone here. Very confusing.
wdecho: I had made up my mind I wasn't going to respond to any more of your posts after the way you have abused everyone, including myself, trying to help you. But for the sake of other readers who might be confused by your latest missinformation I want to set the record straight.
(1) The Klipsch HIII is the best speaker I have heard in all of the speakers I have auditioned, but yes, it is still a little harsh on my ears at times. I plan to keep them because they do so many other things well.
(2) I believe L-pads are an *excellent* way to tame "too hot a driver". I explained this to you in this thread and months ago in a similar thread. The point of all the math was to demonstrate to you that the L-pad will change the load the crossover sees (an assertion you seem unwilling or unable to recognize). I never made the claim it was "the wrong way to do it", just that it wasn't totally inert as you claimed. Hear this. I believe that although the L-pad will change the load, this change is *very likely not significant* enough to ever be heard, This means I agree with you that L-Pads are good!!!!!!!! Get it yet?
(3)L-pads may in fact be the best approach to too hot a driver. Equalization is another very good approach. I like and use them both.
(4) I don't want to change the crossovers in my new speakers.
(5) Please stop telling people with way more education than you that they need to go to electronics school.
Here is the simplified version of the above:
(1) L-pads are an excellent choice for taming too hot a driver.
(2) L-pads will effect the crossover slightly but you probably wont hear it.
(3) Stop being rude.
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What was this thread about?
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9 hours ago, wdecho said:
You are wrong and do not understand what is happening in the relationship of attenuation and a crossover. You start with a crossover and driver that is rated for a nominal resistance, for convenience say 8 ohms. One can use a meter and measure what the resistance of the driver is if you want to split hairs and design and L-pad to retain that value. In reality it varies with frequency and when attenuating the driver and adding another resistor in parallel with the driver you still retain everything the driver and crossover did before the L- pad was added. You are not changing anything when an L-pad is added that is not already happening before the L-pad is added to the circuit. Think about this. The driver will still have what you are calling "the complex impedance or the driver." That is not going to change by adding an L-pad. Absolutely everything in the circuit with driver is retained and nothing has change except attenuation in a properly designed L-pad.
Sorry Mike, I do not mean to be argumentative but the record does need to be set straight about what attenuation is and how it does not change anything in the design of the crossover. Members need to have sound advice about this matter. There are many talented electronic guys, more talented than myself, on this forum that understand what I am saying is correct. Please give it some thought. It is not about who is right or wrong.
For correction, the values I gave are correct when the parallel resistor is added before the series attenuation resistor but in practice it is done after the series attenuating resistor changing the value of the parallel resistor. The value of the parallel resistor when added after the series dropping resistor for a nominal rated 8 ohm driver for 3 db attenuation would be 19.39 ohms. When 19.39 ohms is attached in parallel with the 8 ohm driver the resistance will be 5.66 ohms, ohms law for resistors in parallel. Add that value to the 2.34 ohms it will take for 3 db reduction on an 8 ohm driver and you will come up with 8 ohms, what the drivers nominal impedance was in the beginning changing nothing in the crossover. I will repeat , not rocket science guys, just first week in electronic school. Attached is a diagram of a typical L-pad attenuation for an 8 ohm drive for 3 db attenuation, the amount considered needed for one to hear a difference. I will repeat that when an engineer designs a crossover network the attenuation, if needed, never enters his mind being that it will change nothing. Attenuation is added, using resistors for most X-overs, after the design of the crossover is perfected.
LPad (Driver Attenuation Circuit)
Z = 8 Ohms
A = 3 db
Parts List
Resistors
R1 = 2.34 Ohms 3.21 Watts
R2 = 19.39 Ohms 2.27 WattsYou have the resistance part right but are missing the point we are trying to make about the frequency dependency of the speaker.
The diagram you posted above may help illustrate this as follows.
Let's call the impedance of the speaker be Z3.
Now the input resistance to this network that the crossover sees (on the left side) is
Zin = R1 + (R2 * Z3)/(R2 + Z3)
Now using the values indicated in the diagram for R1 and R2 we get
Zin = 2.34 + 19.39*Z3/(19.39 + Z3)
Now if we let Z3 be exactly 8 ohms, solving for Zin gives
Z3 = 8
>> Zin = 2.34 + 19.39*Z3/(19.39 + Z3)
Zin = 8.0034
Which is almost exactly 8 ohms. Which is the goal of this properly designed L-pad,. (This is your point I believe)
But this speaker is not always 8 ohms. Its actual impedance changes with frequency.
Let's suppose that at a certain frequency the speaker's impedance is 16 ohms, then re-evaluate the same equation setting Z3 to 16.
Z3 =16
>> Zin = 2.34 + 19.39*Z3/(19.39 + Z3)
Zin = 11.1063
Now we see that the load the crossover sees is approximately 11 ohms, but if the speaker was connected to the crossover without this L-pad it would be 16 ohms.
Hence the introduction of the L-pad changes the load presented by the speaker on the crossover.
Hopefully this helps clear things up.
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14 minutes ago, mikebse2a3 said:
if you alter the frequency dependent impedance of the network from the original design (with the understanding it's component values were optimized with that impedance in mind also) then you very likely compromised the performance of the system.
Mike is right. The combination of the speaker's complex and frequency dependent impedance and the resistor's real and constant impedance will add to produce something different than what the crossover was designed for. I doubt anyone could hear any ill effects of the L-pad, but yes, it is definitely going to be a different load.
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For anyone on the fence about tone controls.
One thing I like about the Schiit Loki 4 band tone control is that is is completely transparent to my most critical listening. And I listen very critically. In general, I don't favor EQ or messing with the sound, but I have got to give some praise to this little device. It greatly exceeded my expectations. By "transparent" I mean that when it is in the circuit and all 4 knobs are at zero I hear absolutely no difference between a direct path and the path through the equalizer. I performed this test using an A/B switch box on the RCA lines to instantly bypass the unit. I can not hear any difference between the direct path and the path through the zeroed out Loki box. That is a good starting point. First do no harm! EQ later.
For comparison, I did the same test with the miniDSP HD unit (also with all EQ functions defeated) and I could definitely hear a difference. When the miniDSP was in the circuit (even though it was set to no EQ) it degraded the signal quality slightly. Very slightly, but I could hear it. I sent it back. The miniDSP HD is an amazingly powerful device, but I didn't really need the full blown room EQ, and didn't want any degradation whatsoever that I could hear.
When I begin to turn one of the knobs on the Loki away from flat I hear it smoothly and almost imperceptibly begin to effect the sound, and with a simple turn back to zero it is invisible again. The Loki is fully analog. This makes a big difference. There is no added A/D - D/A stage to mess things up like some other EQ boxes. So if you are looking for just a little more bass at low volume or moving the mids a bit back in the mix when you get a recording that is not so well recorded, but are not sure you want to "mess up" the sound with tone controls I suggest giving the Loki a try. You may find it as sonically transparent as I did. It also has it's own built in bypass switch. I dare anyone to tell me you can hear when this thing is in the circuit.
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Sound bar
EDIT: With L-pads
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The music we are playing back has already passed through an awful lot of Op-amps and equalizers before we ever get it.
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8 hours ago, wdecho said:
I have bought from Parts express. I usually just buy 15 watt ones for horns for around $6 apiece. Horn speakers at home will never see a continuous 15 watts. Probably more like 2 or 3 watts on peaks.
I have two of those same L-pads from Parts Express at home. I didn't realize the speaker backs were screwed on. I didn't notice screws.
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Agreed. If I built speakers I'd put an L-pad on each tweeter and mid just because it's easy and cheap (and I like tuning stuff).
Will the mid horn in the HIII respond well to an 8 Ohm L-pad without it messing up the crossover frequencies? There seems to be a lot of debate about such things.
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The closer I sit the better these sound. I am really enjoying what I am hearing lately. I think that when too far away the room messes up the sound.
Right now the HIIIs are about 6 feet away and 6 feet from each other, on the floor on their original risers and it is very nice.
(Compared to maybe 20% on the volume knob of my same wattage Marantz 1060) No clue 
New VTA ST-70
in Talkin' Tubes
Posted
I tend to agree. Every amplifier has a characteristic sound. It seems that the question comes down to do you which sound you like better, the preamp, or the power amp? From what I'm reading it appears most of the big money is invested in preamps to get a super great sounding but very fragile signal, and then the power amp is just about making this louder and strong enough to drive speakers.
So using a path into the power amp that is as transparent as possible only has value if indeed the power amp is what you want to hear.