Platenspeler.Com |
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Platenspeler.Com |
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First lets build a passive "amp". I use quotes here since there will be no amplification at all, it's merely an input selector followed by an attenuator.
For the 3-step switch I chose a simple model from Lorlin (UK) with 4*3 steps. That means I'm able to switch both channels AND the ground signal and still have one range of contacts left for future use, for example to switch LED's that indicate which channel is active.
I've chosen three inputs, this should be sufficient in most cases. More is always better but in this case the costs go up significantly if you go to the more sophisticated models.
Passive or active, we need an attenuator for control the volume. keeping in mind that in the end we want an universal pre-amp for Gainclones usable for non-inverted configurations than an attenuator with an impedance of 20-50k Ohms is ideal. Of-course it's possible to use a pot here as well, an ALPS blue of 50k is a good choice for anyone not willing to do he soldering. However, I like a 12-step attenuator very much and it's easy to build with a cheap switch and 24 resistors.
For passive usage with gainclone applications 20k is the better choice, but for active buffering a value of 50k is better. In anticipation of the active buffer I'm going to make a 50k stereo attenuator since I expect to "upgrade" this project to an active version later on.
|
Step |
from |
to |
Resistors |
dB attn |
dB step
|
Impedance |
| 1 (low) | GND | 1 |
82 |
-55.66 |
--
|
49,742
|
| 2 | 1 | 2 |
150
|
-46.62 |
9.03
|
49,742
|
| 3 | 2 | 3 |
330 |
-38.94 |
7.68
|
49,742
|
| 4 | 3 | 4 |
680
|
-32.05 |
6.89
|
49,742
|
| 5 | 4 | 5 |
1,000
|
-26.92 |
5.13
|
49,742
|
| 6 | 5 | 6 |
1,800
|
-21.80 |
5.12
|
49,742
|
| 7 | 6 | 7 |
3,000
|
-16.98 |
4.82
|
49,742
|
| 8 | 7 | 8 |
4,700
|
-12.54 |
4.44
|
49,742
|
| 9 | 8 | 9 |
5,200
|
-9.36 |
3.18
|
49,742
|
| 10 | 9 | 10 |
6,800
|
-6.42 |
2.93
|
49,742
|
| 11 | 0 | 11 |
10,000
|
-3.37 |
3.05
|
49,742
|
|
12 (hi) |
11 | 12 |
16,000
|
0.00 |
3.37 |
49,742
|
|
49,742 |
56.00
|
This is the design for the attenuator, and when finished it looks like this:
Making your own stepped attenuator is quite easy, read on the background page how to design your own should you wish to make one with a different input impedance.
Well, this project made it a lot easier to use gainclones on a daily basis. I need an amp with more than one input: The netgear, a cdplayer and a phonopre all need access to the clone and I hate switching connectors all day just because I want yo listen to another source of music.
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