# QA403 attenuation not working? **URL:** \ **Category:** Uncategorized\ **Created:** [September 6, 2024, 9:38pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545 "2024-09-06T21:38:06Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![RAW-CAt](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/raw-cat/32/2733_2.png) [@RAW-CAt](https://forum.quantasylum.com/u/RAW-CAt)\ **Post date:** [September 6, 2024, 9:38pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/1 "2024-09-06T21:38:06Z")
I’ve encountered an issue. Trying to measure a monoblock 2000W@2ohm amplifier. Whenever the voltage goes above 17V or so, the input of the QA403 starts to clip. I verified with the scope that it’s not the amplifier. The input of the QA is set on +42dBV. Usually I use REW, but this time opened the QA software and have the same results. Just a few hour previously I measures successfully up to 28V. I made my connections the same as I dis countless other times… Any ideas? ![1000013284](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/e/ed4a63c1a178e31fb3b3b1a3fbc528e990c7e4a4.png) ![1000013283](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/4/4559b5c1bae0eeb89868ac69641767eda6c93edd.png) ---
**Author:** ![restorer-john](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@restorer-john](https://forum.quantasylum.com/u/restorer-john)\ **Post date:** [September 6, 2024, 9:58pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/2 "2024-09-06T21:58:28Z")
How are you connected to the amplifier (balanced or SE)? Is it a BTL amp or single ended? Is it a BTL class D with DC offset with respect to “ground”? Also, clearly you are using an external attenuator to measure such powers (2000w@2R). How is that configured? ---
**Author:** ![RAW-CAt](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/raw-cat/32/2733_2.png) [@RAW-CAt](https://forum.quantasylum.com/u/RAW-CAt)\ **Post date:** [September 6, 2024, 10:24pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/3 "2024-09-06T22:24:36Z")
The connections are single ended. I am not sure if the amp is BLT (facepalm). No external attenuator is being used as I want to measure only up to 40V as the QA can. ---
**Author:** ![RAW-CAt](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/raw-cat/32/2733_2.png) [@RAW-CAt](https://forum.quantasylum.com/u/RAW-CAt)\ **Post date:** [September 6, 2024, 10:52pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/4 "2024-09-06T22:52:00Z")
Ok I figured it out. The amplifier is BLT and I need to use a probe with differential connections to the QA. All is sorted and now everything is working. Mod, feel free to delete the thread. ---
**Author:** ![restorer-john](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@restorer-john](https://forum.quantasylum.com/u/restorer-john)\ **Post date:** [September 6, 2024, 10:56pm UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/5 "2024-09-06T22:56:12Z")
If the amp is BTL (PS, just had a BLT for breakfast!), you’ll need to use the differential inputs, otherwise you will likely drive one leg of the amp into a short (BNC outers are all common on the QA403). ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [September 8, 2024, 12:07am UTC](https://forum.quantasylum.com/t/qa403-attenuation-not-working/1545/6 "2024-09-08T00:07:39Z")
> Mod, feel free to delete the thread. Hi @RAW-CAt, I’ll leave in place if you don’t mind. this is a fairly common problem and bit me the first time measuring class single-supply D push pull. I just posted some spice simulations at the link below to help illustrate this. In short, when measuring a push-pull amp, you probably want a 3-resistor attenuator. But when measuring a single-ended amp, you probably want a 2-resistor attenuator. > [@QA402 Input Range Violations using Differential Inputs](https://forum.quantasylum.com/t/qa402-input-range-violations-using-differential-inputs/1486/31): > > Hi @cfortner, overall this looks like a solid setup. But there is a bit of a wrench lurking inside the setup that isn’t surmountable, but needs to be understood since you are working towards a universal setup for all topologies. Let’s start with SPICE to understand the external divider. In the picture below, I’m using a voltage controlled current source to generate a balanced signal. This is TI’s preferred way of nudging SPICE to do what is needed. I wish I could find the TI engineer’s name tha…