[raindance]

If you turn the CD player output up all the way and it is not loud enough, then you'll need a preamp. Same problem I have. I got myself an SDS-254 and it solves this problem nicely as it has variable input gain.

Most people here are running the CDA amps with active preamps.

Evidently you can send the amp back to Tom to get the gain changed but if you bump it up the input impedance drops. You don't need that, it is already really to low for practical purposes.

I've found that to really get the treble detail that these amps (CDA series) are capable of I have to at least buffer the input with an op amp.

[nwboater]

If you turn the CD player output up all the way and it is not loud enough, then you'll need a preamp. Same problem I have. I got myself an SDS-254 and it solves this problem nicely as it has variable input gain.

Most people here are running the CDA amps with active preamps.

Evidently you can send the amp back to Tom to get the gain changed but if you bump it up the input impedance drops. You don't need that, it is already really to low for practical purposes.

I've found that to really get the treble detail that these amps (CDA series) are capable of I have to at least buffer the input with an op amp.

This alone may justify the extra cost of the SDS series. Yes?

Rod

[raindance]

Yes, I would think so. Although in my head the jury is still out on which sounds better. Right now, with my home made buffer with no gain, the CDA sounds much better but won't go loud enough. The SDS without my buffer sounds a bit thick and rolled off in comparison but goes plenty loud.

[roymail]

The SDS without my buffer sounds a bit thick and rolled off in comparison but goes plenty loud.

Have you tried driving the SDS with a 10K or 20K stepped attenuator?

"Thick and rolled off" sounds like the value of the passive unbuffered volume control is 50K to 100K.  I'd be interested to know if that's the case. 

[Occam]

Yes, I would think so. Although in my head the jury is still out on which sounds better. Right now, with my home made buffer with no gain, the CDA sounds much better but won't go loud enough. The SDS without my buffer sounds a bit thick and rolled off in comparison but goes plenty loud.

Raindance,

Consider changing the feedback resistors on your LME49710 opamps to give you 6-12 db gain.

FWIW,
Paul

[waltsok]

Would bridging two CDA amps solve the volume issue by any chance?

Are you suggesting that by buying the SDS I would have sufficient gain? If so maybe I'll try selling the CDA and replace it with an SDS if it's sound is similar. I don't like the thought of thick and rolled off, however.  If I get an SDS I should get a buffered passive too? Usually when adding something, a preamp for instance, doesn't that muddy the water?

 I like the idea of building a preamp maybe with the above mentioned LME4970, if it sounds as good with additional gain. I would need guidence though. Anyone game?  Thanks Walt

[raindance]

OK everyone. My buffer was a non-inverting stage with 100% feedback. The only resistor in that circuit is between input and ground and it sets input impedance to a sane value. On the LME chip that sounded great. When I rewired to an inverting gain stage with a gain of 2 it did not sound good at all. Of course I reversed my speaker wires to keep things in phase when I tried this... This is possible a quirk of this chip. Also note I was running off a single supply and this is not the best way to reject power supply ripple, although my supply was regulated. Possibly you would get better results with a split supply, I just did not have a transformer lying around for that plus Ratshack doesn't know what negative regulators are 

@Roymail: yes, I was trying to use a 100K motorized Alps black that I ordered for another project. Padding the value down with resistors would totally screw up the curve of the pot, so I don't really want to go there. Buffering it is the key to avoiding the "thick & rolled off" description.

So, regarding the CDA series: there is great benefit from buffering (no gain) the input to make it an easier load for your preamp. But you'll need a preamp with gain.

Regarding the SDS series: drive it with a low value pot, around 10K if you have to go passive, or use a preamp or buffer with gain or no gain, your choice - the gain pots help sort this out. Or use a no gain buffer like I suggest for the CDA if you have to drive it from a high resistance pot.

[raindance]

10 Ohms gives you basically as much input sensitivity as you'd get if you left the volume controls in place, and cranked them all the way up.  I've been using values around 5K ohms, and that drops the input sensitivity by about 6 decibels (which works really well with my Pass DCB1).  I've tried 1/2 watt and 3/4 watt and even one 1 watt rated resistor and that doesn't seem to make any difference at all.  I guess I'd suggest using 1/2 watt resistors unless you can't find the impedance value you want at that rating, at which time I'd feel free to go up a bit.

OK, I volunteered via a PM with another member to get some info from Tom (the amp designer) on the gain resistors/pot situation. Here is his response:

First, the amps are AC coupled (EDIT BY RAINDANCE - So you can try them with your Little Dot Mk III if you want). The gain is in the first stage of the amp.

 

I was very careful in selecting the pots that come with the amps. They feature high quality conductive elements and are pretty much dead quiet. Of course, you can use stepped attenuators for possibly a little better performance, but most could never be able to hear the difference. These do not come with the kits because as I’m sure you already know, you can spend allot more money on quality stepped attenuators than the cost of the amp kit. Fixed resistors are another and possibly the best choice as long as you know the correct gain to set the amp at to get the most out of your preamp or other input device. Without taking this into account and just changing to a fixed resistor, it is possible that you won’t achieve the optimum sound quality from your equipment if your preamp is not matched correctly to the amp and adjusted to find the maximum and optimum sweet spot of the preamps volume control. Distortion from the preamp can be a problem on many units if the volume control is turned up too high, and you also might not be getting the best from your preamp if the volume is turned too low… they are all different.

 

To put it simpler…

 

The gain controls are used to match the amplifier’s gain to the gain of the other amplifiers in the system as in active or passive preamps, DAC, iPod or mp3 players, etc. The gain controls allow you to match the amplifier to the input device. Not all input devices have the same maximum preamp output voltage. Some preamps are capable of producing 15 volts RMS out while others are only capable of 1.5 Volts RMS out. Most preamps or other input device will reach its maximum output level (clipping) well before the volume control reaches the upper end of its range, usually at a point of 75-90% of its maximum range.

 

For those who wish to change the pots for fixed resistors, here are resistor values to produce required gain. Gain is listed in dB: To change gain potentiometer to a fixed resistor, you would connect a fixed resistor between the yellow and black wires on the gain controls (of course, remove the gain pots). Leave the red wire disconnected.

 

Use high quality 1% resistors:

 

560 Ohm = 31 dB

 

1K = 30 dB

 

2.2K = 27 dB

 

5.2K = 23 dB

 

6.2K = 18 dB

 

7.5K = 16 dB

 

I hope this is of some help. Please feel free to post this on the forums if it will help others. On my personal systems here, I use the same pots that come with the kits.

[raindance]

Buffer circuit, non-inverting, using any op amp WHERE THE SPEC SHEET SPECIFICALLY SAYS IT IS UNITY GAIN STABLE. Note that this uses a split supply and can be DC coupled both ends and I only show one channel. This circuit is the most simple circuit in the op amp world and works as an impedance transformer. The input resistor sets the input impedance. It is that simple. The output impedance is low and will drive the CDA series 7K load easily.

The decoupling caps have to go as close to the power supply pins on the op amp as possible and the power supplies must be regulated. Use cheap electrolytic or tantalum caps for the larger value (this is cheap & cheerful after all) and ceramic or other non-polarized small caps for the low value.




If you want more info, I can add the power supply and also some detail on how to do this with a single supply. This is all basic electronics, nothing fancy. Always check for DC offset at the output before trying a home made buffer with your system! Also, make sure the buffer is on before the power amp to be safe (protect your speakers from turn on pops).

[raindance]

Roymail just sent me a PM that reminded me of something:

With the SDS amp you can replace the two 10K "gain" pots with a 10K dual log volume control and you are DONE. You have an integrated amp with one input. This would probably be the best sounding configuration as well.

Don't do this if you are using a preamp that has gain, do it only if you want to run totally passive.

I'm betting this will be the BEST way to run these amps.

[raindance]

Single supply version of the buffer (note that this works fine but has poor PSRR due to the fact that you can't place any significant size cap on the divider to bias the op amp halfway for single supply use - the cap forms a low pass filter).

Input impedance is 100K in parallel with 100K which is 50K. Don't go too high on the divider resistors as the biasing won't work.

[pelliott321]

there are no pots on the CDC so I this means they have to go back for the change?

[poseidonsvoice]

For those who wish to change the pots for fixed resistors, here are resistor values to produce required gain. Gain is listed in dB: To change gain potentiometer to a fixed resistor, you would connect a fixed resistor between the yellow and black wires on the gain controls (of course, remove the gain pots). Leave the red wire disconnected.

Use high quality 1% resistors:

560 Ohm = 31 dB

1K = 30 dB

2.2K = 27 dB
 
5.2K = 23 dB

6.2K = 18 dB

7.5K = 16 dB

I hope this is of some help. Please feel free to post this on the forums if it will help others. On my personal systems here, I use the same pots that come with the kits.
[/color]

Extremely helpful post. Muchos gracias Raindance.

Thanks!

Anand.

[raindance]

there are no pots on the CDC so I this means they have to go back for the change?

What are you trying to find out?

[pelliott321]

I had mentioned a few days back that I was disappointed with the CDA I just purchased in that it was very dull sounding and had no gain.  The discussion centered on my tube preamp that has 700 ohm output impedance.  I believe you suggested an input buffer and it was also mentioned that the importer can change the gain and that might be the solution so I just wanted to confirm that before I contact him

[roymail]

It shouldn't be this hard.  Once again I recommend that you contact Tom.  He's very helpful.  The problems you're having don't seem to be typical of these amps.

[raindance]

According to Tom, he can change sensitivity on the CDA series. Decreasing sensitivity INCREASES input impedance, making the amp more tube friendly, but you'll have to be sure your preamp can swing enough volts to drive the amp properly. You already feel you have no gain, so I doubt this will help you. Going the other way will decrease input impedance and will probably not work with a tube preamp.

Also, which tube preamp do you have?

[pelliott321]

I using the transcendent sound Grounded Grid
Specifications:
Harmonic Distortion:  Less than .02% at 2 volts RMS out.
Signal to Noise Ratio: Greater than 93 dB.
Bandwidth:  1 dB down from 5 Hz to 300 kHz.
Gain:  12 dB.
Max Signal Out:  20 volts RMS.
Output Impedance:  200 ohms.
Input Impedance:  50 k Ohms.
Inputs:  3, Outputs:  1, all single-ended, RCA
Controls:  Input Selector, Volume, Power
Tube Compliment:  3-12AU7
Dimensions Overall:  15'W by 11'D by 4 1/4'H.          100-120V, 200-240V, 50-60 Hz, 25 watts
Weight:  10 pounds.
 

[poseidonsvoice]

My SDS 254 build with pictures is here for anybody interested. Questions/comments/criticisms welcome.

Anand.

[Nick77]

My SDS 254 build with pictures is here for anybody interested. Questions/comments/criticisms welcome.

Anand.

Anand your build looks amazing. You certainly took some extra measures in regards to power, care to comment on your sonics compared to the stock unit you demo'ed?