Running the Crack without a volume control

[CrackMeUp]

Hi everyone,

This is my first post on the Bottlehead forum. I love building things hifi related but so far have only made a DC blocker, modified a few things and made some leads, so the Crack will be my first full build. Before I take the plunge, however, I'd like to know whether it's okay to run the Crack wide open with no volume pot or stepped attenuator. The reason is that I already have a very good TVC in my system - the Music First Audio Baby Reference preamp. I'd like to control the headphone volume by adding another pair of RCA connectors and a switch to the TVC so I can switch the output between amps/speakers and the Crack/headphones. Would this setup work okay with the Crack? Although the Crack would have no volume control, the current from the selected source would still be attenuated by the TVC so the Crack wouldn't be run anywhere close to max, and no gain is added by the TVC.

Thank you!

 

[AB2KH]

CrackMeUP,

What I would do.  I would build the amp as per the Bottlehead manual. Install it in your system with your desired volume control  (passive / Active) and just turn the crack vol control all the way up effectively removing it from the audio chain, that way if at some point you decide to use it with it's own dedicated source as a stand alone system you can do so.

Chris/AB2KH

 

[CrackMeUp]

Thanks Chris. That's sound advice, of course, but I can't see me wanting to use it outside of my main system, so just wanted to know if it's okay to bypass the volume pot entirely. Any info on how to do that would also be useful if anyone knows what to do and/or has tried it.

 

[Thermioniclife]

If you do not want the volume control I would solder a 100k ohm resistor from the grids of the 12au7 to ground so the 12au7 has a grid reference to ground even if you do not have any input connected. This will keep everything happy.

 

[AB2KH]

I personally would loose sleep over the now hole in the chassis, but then that's just me,having worked in an industry which required your product to be visually perfect.

Chris/AB2KH

 

[Paul Birkeland]

Just turn the pot up all the way.  The potential problems you could run into by trying to leave the pot on are going to be a significant bother compared to just building it stock.

 

[CrackMeUp]

If you do not want the volume control I would solder a 100k ohm resistor from the grids of the 12au7 to ground so the 12au7 has a grid reference to ground even if you do not have any input connected. This will keep everything happy.

Thank you. My thinking was heading in this direction so it's good to hear someone else say it for confidence.

I personally would loose sleep over the now hole in the chassis, but then that's just me,having worked in an industry which required your product to be visually perfect.

Chris/AB2KH

I would leave the pot in place in case i did ever need to wire it in, just not connect it for now. That way the aesthetic would also be satisfied.

Just turn the pot up all the way.  The potential problems you could run into by trying to leave the pot on are going to be a significant bother compared to just building it stock.

Thanks Paul, but could you please expand on the 'potential problems' I could run into, and what you mean by 'trying to leave the pot on.' Isn't that the same as turning the pot all the way up (on/open) as you suggest?

Thanks for all your replies. I really appreciated it. The main thing driving me to want to do this is that I like to keep the signal path as clean and simple as possible. With my monoblock power amps, the signal just connects to the grid of the input tube and uses a grid leak/drain/ground resistor where a volume control might be. My power amps don't have volume control, so I was thinking the Crack could be run in the same way without issue. I'd just like to confirm that's the case before I dive in.

 

[Paul Joppa]

A couple comments:

A) The volume control IS a 100K resistor, so merely connecting the grid to the top of the control instead of the wiper accomplishes the fixed gain without removing any parts, and with  minimal wiring changes.

B) If you do that, you are likely to discover that you have way too much gain in the system, i.e. a hair-trigger TVC and excesssive noise.

 

[CrackMeUp]

Thanks Paul, so no other issues other than the potential for too much gain. I'll certainly build it standard to start with, but I just know that pot will bug me. My TVC has small steps for about half of the 32 positions on the volume dial, so it's a finer tuning for the lower levels. I guess it's down to suck it and see. Just one more question then... If I find it's okay with the Crack's volume at max, and I have good volume control with the TVC, I can just bypass it with 100k resistors to the input tube grids, and if it's too much, could I try higher value resistors to effectively reproduce running the crack at a lower volume? Thanks again.

 

[Paul Birkeland]

Thanks Paul, but could you please expand on the 'potential problems' I could run into, and what you mean by 'trying to leave the pot on.' Isn't that the same as turning the pot all the way up (on/open) as you suggest?

PJ brought up the gain issue, but the other issue is that the ground buss routes through the pair of ground lugs on the volume pot, and when you lose those it will make the Crack much harder to build and possibly less reliable.

 

[CrackMeUp]

PJ brought up the gain issue, but the other issue is that the ground buss routes through the pair of ground lugs on the volume pot, and when you lose those it will make the Crack much harder to build and possibly less reliable.

Thanks Paul, so I'd just need to ensure I continue the ground wiring so ground still gets to where it's supposed to go, but without the using the pot lugs as a tags (or I guess I could still make use of them if I leave the pot in place without the signal wiring connected). I plan to build it per the manual first, so I'll be able to see where everything needs to be connected up with the pot gone, and that it all works ok. I think I'm getting there. So, I just the potential gain issue to worry about really by the sounds of it. Signal wires go to the respective input tube grids and a 100k resistor from the grids to ground bus. Then if gain too high, drop a quality resistor in the signal path emulating the pot resistance at whatever volume level works best. Does that sound right? Sorry for all the questions. About to buy one. Just want to make sure I can achieve what I'm after with it.

 

[Paul Birkeland]

The idea of moving the wires from the middle lugs of the pots to the outer lugs of the pots is a winner.  I would build it stock first, then do that if all is well.

 

[AB2KH]

There was a gentleman by the name of Rube Goldberg who was extremely adept at this very type of of problem solving, if he were still living I'm certain that he would have the ultimate solution to this.

Chris/AB2KH

 

[CrackMeUp]

Thanks everyone for your replies and input. I've just ordered my first Bottlehead kit - the Crack 1.1 + Speedball. Can't wait to receive it. It's a Christmas present from my wife, so I'm keeping my fingers crossed that Santa will be able to deliver it in time.

 

[Deluk]

I've always thought that the last few percent of an amplifiers gain wasn't the best place to be if it's in the sound chain. This is where distortion and extra noise hang out. If the crack is maxed out on the pot I'd think that it would also be in this less than best part of the sound. Comments?

 

[Paul Joppa]

There is an optimum gain setting for each item in a signal chain, steering between overload distortion and background noise.

Back in 2007 I wrote a 5-page article on this question; it used to be on the archives/community page but I discovered this morning that the link is dead. Email me paul at bottlehead dot com and I'll send a PDF. It's a little dated but covers the theory well enough.

 

[CrackMeUp]

I have two pairs of tube monoblock amps - 300B PSET and 2A3 SET - and neither have in-built volume control. The signal feed goes straight to the grid of the input tube. In both cases then, the amp is running at it's full potential with no additional resistance in the signal path before the grid. That resistance is provided by the external preamp. So my thinking is that while the amp/s are running at full potential, the incoming signal is still being attenuated. In my case, by a transformer rather than resistors, which I prefer. So the amp still isn't being driven anywhere close to max at normal listening volumes. It's going top be fun to try a before and after test, but I'm pretty sure that controlling volume with an external TVC is going to yield improvements, just as, I'm sure, the BH stepped attenuators do. I don't think it matters whether the VC is in the same box as the amp or at the other end of the interconnect in the preamp if all you're using to control the volume is another stepped attenuator. Looking forward to testing the theory.

Steve

 

[diynewbie]

I'm  having a hard time following this thread.  You all need to to dumb it down for me.

My thinking is that both preamps and amplifiers are fixed gain devices.  Paul Joppa's point is that circuitry and components are optimized to provide the cleanest gain of a range of input voltage to output voltage.  Another way of looking at it is more or less gain would be possible, but it wouldn't be as clean.

Input voltage in the gain stage in a preamp or headphone amp is controlled by an attenuator, which reduces the source voltage to an appropriate level.

I had to look up TVC, but my understanding that it is either a volume control with a step up transformer or a varible transformer with the former being the most likely.  It is an attenuator followed by a transformer that acts as the gain stage.

So, the proposal is to remove the attenuator in the crack and replace it with an attenuator  followed by a step up transformer. (the TVC).  The attenuator in the TVC has to reduce the source voltage to remove the gain from the transformer and then reduce it further to obtain the appropriate level of voltage for the gain stage in the crack.  Is this correct?

 

[Paul Joppa]

TVC is usually a variable step-down transformer (or autoformer...).

 

[AB2KH]

Good Day All,

1 Purchase kit (this will help Doc with his salt flat racing program)

2 Assemble kit AS YOU DESIRE (If you deviate from the manual you are on your own)

3 Pass resistance tests

4 Pass voltage tests

5 Instal in your system AS YOU DESIRE

6 Turn amp ON allow it to warm up

7 Pore Bourbon

8 Select headphones

9 Select music file

10 Press PLAY, sit back and enjoy

This is not complicated stuff

  Chris/AB2KH

There is no cure for Audiofilianervoasa, however, thankfully, it can be effectively managed.